My horse was 32 when he was sadly pts on Boxing Day, so I know about looking after the oldies. Yes get his teeth checked, I would recommend getting a specialist horse dentist rather than the vet to do it, they will do a much more thorough job

Also make sure his worming is up to date.
Is there anyway you can seperate him with one other that he gets on with to save him getting bullied? Old horses tend to get bullied, it's just herd mentality to push out the old and weak ones sadly. My old boy was bullied at a livery yard about 6 years ago, so I had to move him to a farm with just him and a pony I had on loan.
Winter is the time where every oldie losses weight. They do tend to lose weight despite your best efforts, and it is an ongoing battle to keep it on them, but it is possible to get them through the winter with just a little weight loss. Make sure he is well rugged up, if he is using energy to keep warm he will lose more weight. Give him a feed with sugarbeet, mix (I always used badminton veteran mix with mine), flaked barley (barley is great for weight gain), and chaff if you can. Add extra oil for more calories. I also fed mine winter glow/summer shine supplement which I swear by. Also carrots and apples, or other veg added. Give plenty of hay or haylage (haylage has more calories and often easier for the older ones to eat).
I would also give mine homeopathy tablets, nothing to do with weight but helped an awful lot with his stiffness, ruta grav and rhus tox, also graphites to help prevent mud fever.
I would feed mine a hard feed all year round, obviously not so much in the summer and only one feed a day rather than two in the winter.
Mine was out during the summer (unless it was very hot, then he was in during the day as he didn't like the flies and got sunburn as he was half appaloosa so had lots of pink skin on his face), but in the winter he was stabled at night.
Looking after an oldie can be rather costly, but mine had given me years and years of joy and he was my best friend, so I would give him the world. I still miss him now x