TPUP - first of all - let me say what a great looking appy you have! I have a 26 yo appy that I have had since he was 6 - his build reminds me A LOT of your horse. DS is not at all the standard Appy build - very long necked, long legged, deep chest but if you ignore the spots he looks more thoroughbred than App. I don't know if you have ever pulled your appy's pedigree, but you might be surprised - my app is only a tiny portion appaloosa (racing app lines) - he is about 50% thoroughbred & 25% or more racing quarterhorse. There is nothing foundation looking about him except his color!
Okay, that said, I don't think your boy looks bad at all for his build, just MHO. From personal experience I can tell you my app has always been a challenge to keep weight on, and it is not grain that puts the weight on for him - it is forage/hay. When I used to board him, I knew within 2 weeks if they had cut back on round/squaare bales or changed suppliers - his weight would drop like a stone. Now that he is older, it is even harder to keep weight on him. This spring (before I was finally able to move him home to my new farm in KY) he was REALLY thin after making it through the winter in SC. The stable in SC didn't have much grass, but my pastures here are very good and after carefully transisitioning him, he is slowly gaining back some weight - but not nearly as fast has the 8 yo TWH I have turned out with him. Here in KY he is on 24/7 turnout and walks slowly grazing around the pasture (about 2 acres) and pretty much ignores the good grass hay I keep out for him under the turnout shed for when it is too hot to be out in the field. I feed him SS Solution 14/6 which he really likes, and eats happily. It can be moistened but so far my dentist has said his teeth are fine, and he has no quiding, so I haven't been making it into a mash.
IMO - the only way you will get weight on him is to reduce the amount of exercise he is getting moving around, and putting substitute forage in front of him - soaked beet pulp, alfafa cubes, etc. If he is having chewing problems in the back, then giving him even grass hay is possibly inviting impaction colic - so I would only feed a soft, easy to chew grass hay when you were sure you were keeping his gut wet.
Overall, I would suggest you judge him as I do my guy - he's not ravenous, he has plenty of energy for work and seems happy in his job and with his life - horses are like people to some degree, some of Jack Sprat, some are Mrs. Sprat! LOL Good luck to you both <3 those APPYS!