Sorry Aesthetic, but I would consider him skinny in the second pictures you posted. He is scoring about a 3 on a body scale, and getting closer to a 2. I know you are doing what you can, and nobody should be judging your horses happiness. If you didn't care you wouldn't be here! So here are my suggestions:
1) FORAGE, FORAGE, FORAGE. Do whatever you can to get him on 24/7 hay...it will make the world of difference. There is absolutely no point in pumping him full of more grain, especially if he does have ulcers, which only increases stomach discomfort and contributes to weight loss. Keep his tummy happy with high fiber and high fat. Feed oils if you can but keep it to a minimum as it can give them diarrhea.
2) Keep riding/work to a minimum until he's back up to speed. Burning much needed calories won't be helpful and muscle is much harder to build without some fat on them first. Stretches and proper body carriage exercises will do wonders for you guys too. He is very "upside down" in the way he stands. Teaching him to reach down, stretch his topline, and use himself BACK to FRONT. All of this can be done on the ground before even stepping foot in the saddle.
3) Definitely test for ulcers, parasites, etc. I would pull blood and do fecals before you treat, that way your not wasting time/money on self-diagnostics. Pretty simple tests and you will feel a ton better knowing whats going on.
Again, these are just my personal suggestions, so take them with a grain of salt. EVERYONE cares for their horses differently. Below are a few pictures of a gelding I bought at the age of 4 who literally drove me crazy trying to keep weight on. I did everything I could think of for him and the thing that finally worked was giving him a good quality round bale of grass/alfalfa to go to town with...results were unbelievable. I had him on hay 3-4 times a day before that, but the 24/7 access really made a difference.
When he came... mag20.jpg
After only a week of free-choice hay hay.jpg