He looks like fine gentlemen and is getting lots of love.
His weight is improving and he's coming along nicely. It takes longer to put on weight than it does to take it off. The weight probably came off quite fast due to the pain from the founder. It does take patience. Keep taking pics to start a timeline and enable yourself to see the improvements and monitor the process. Sometimes you have to look back to confirm that you are moving forward. As long as his body is poorly, it will getting priority with him, not his feet. So you'll need even more patience to get it all to shine.
I looked at the Firko feeds. Not sure which one it is that you are giving him and how much/day. I'm also not sure of the amounts in it. Are you near Greece? At any rate, the feed doesn't have an NSC content, which is the sugar that's in it. You could phone them and ask, but since its got molasses in it, just know its high. You should be avoiding molasses. That includes molasses-free beet pulp as well. But since this feed has some important B vitamins/probiotics, I'd keep it around for awhile until he gets over the hump. I noticed that the trace minerals are added to the feed and their amounts are not balanced. Iron is another enemy and zinc and copper are not sufficient to fight it. If you can find some minerals (powder) that you can set out for him. He will decide what his needs are. I would add the beet pulp to his diet now. In terms of laminitis, this food is the safest way to weight gain that you can get. I would start with a 1/2 cup of beet pulp soaked and rinsed, which will expand to 4 cups when ready and add it to his dinner. You are soaking and rinsing it to remove iron and sugar and easier delivery to avoid choke, so don't feed it dry. Adding a final amount of water to it, to make the whole thing soupy will also add water to his diet. When his weight gets over the hump, I"d wean him off the firko and keep the beet pulp.
Chances are, his story is that he was left on pasture and forgotten and it was the grass that did this. I would let him graze when you hand walk him, but not turn him out indefinitely. Make the bulk of his diet hay, as it is safer as well. If you get him padded well and he's looking more comfortable in his movements, I would incorporate small hills to build muscle. When not walking, spread his hay out in different areas to encourage him to keep moving and the padding working for him. That's his homework.
Regardless of what you are wrapping his hoof in, its the padding below the hoof that is supporting the bone that is important. Whatever you can get your hands on...children's rubber play mats, and old gel saddle pad, your old rubber flip flops. If he's not comfortable in one layer, double it. The padding is magic. It provides a soft landing and bottoming out of his weight on that bone, then supports and coaxes the bone to get back up where it belongs...with every step. It's an important promotion for a low P3.
I wouldn't be riding him yet. He's having trouble supporting his own weight, let alone yours as well. I would also not be lunging him or making sharp turns. The walking is for a mental lift and making him take those steps to get the padding to promote....walking him for health reasons, not training reasons.
I would also be feeding him flax seeds, which must be ground just before you feed it. I have a small $10 coffee grinder at the barn which works well. 4-6oz./day. All feeds should be added by weight, not scoops. These are your omegas, the only fat he requires. It has a perfect balance of omega 3 to 6, which works as an anti-inflammatory and will help with pain as well. The 3 is the anti-inflammatory, the 6 is in the flammatory. Black oil sunflower seeds are high in 6 and promotes inflammation, so no to those. You can also get Chia seeds that are good and you don't have to grind them.
If you find yourself so limited as to feeds provided at the feed store, then turn to human ingredients. Give him the same vitamin A that you would take and up the E. Get the flax or chia from the grocery store. Get in touch with mybesthorse.com and ask about Move-Ease and Jiagulan. If you can get it delivered to your door online, then all logistic problems of living on an island will be surpassed. So, if you find the feed store limited, look elsewhere. I would also speak to the vet about the selenium content in your soil and if that needs to be addressed. The vet will have nutritional recommendations that will be better advice than a feed a store and may just have something back at the office he would recommend. You should discuss all this with him in depth. Getting the diet organized and balanced will ensure success.
Laminitis is a symptom showing later in the process that trouble is brewing. He has gone past this stage and gone right on to founder. So he doesn't have laminitis now, but a lot of healing to do. He will always be prone to it now and why you need to watch the sugar and starch content in his feed from now on. So, no molasses or grain or 24/7 grass. The Move-Ease is a biggie for killing pain due to soft tissue damage and the jiagulan will restore blood flow. Its the blood flow that heals....taking the damage away and providing nutrients/oxygen for repair. Its that blood flow that provides impact protection as well.
Ulcers are big concern in this situation. A lot of these recommendations are to prevent them and is another source of pain. The salt to drive thirst, the fiber in the beet pulp, the fiber in the hay and constant intake of it. In a grazing animal, the intake should be constant. Unlike us, a horse's stomach is constantly producing stomach acid to digest. If the stomach is empty, that acid will start eating his stomach instead. Ulcers cause pain and will stop him eating. They are also very expensive to fix, so strive not to go there. Pain is your enemy no matter where its coming from.
So have a nutritional conversation with the vet and ask about the things I've mentioned. Would be a good start. Hope this helps....