Devil's advocate here too, I've been on the opposing end of the discussion. Leftover hay when you're mucking several stalls a day is a real pain in the butt - it's more difficult to remove than urine or manure and even worse if the horse moves around a lot in the stalls and makes a big mess. It may not seem like a big deal to you but barn staff want to be as efficient as possible; this includes taking the time to clean the stall properly and save as much unsoiled bedding as possible instead of stripping it, as well as minimizing leftover hay - you don't want the horse going without hay, but you don't want to overfeed and have tons of leftovers, that's a lot of wasted hay, as well as wasted time. Usually barn staff are paid per stall, and every minute counts for them. If they're paid hourly, then every minute counts for the barn manager and they're still feeling the heat. It's a hard, thankless job, and you get WAY more with sugar with them. Shrugging their challenges off, as small as they may seem to you, will only be met with resistance. While Joe4d might be right about the contract, that's a real good way to tick off otherwise good staff - don't expect any favours from them if that's the stance you want to take.
There are ways to work with them, like monitoring how much hay he is leaving behind and adjusting the amount you give accordingly (hay net helps too if it's easy for them to install), offering to pick out the stall to make up for the leftover hay (some barns will go for this, others won't, many barns are very protective of their equipment, and often boarders would come in, muck their stall, and dump a massive load of bedding in for us to chisel out the next morning). I'm hoping that filling the extra water bucket you put in whenever you're there is a given. Keep the staff happy and they'll be more inclined to keep you happy and put in the extra effort.
In your case there very well could be a management issue, the dropping weight is a concern but he's also not finishing what you're giving him. I have to giggle at the thought of 15 horses on 12 acres being too many though, try 70
Of course, that's a city barn that warrants individual paddocks, feeding, and a LOT of exercise, no round bales. It can be done with good management though. Full boarding just isn't for everyone, glad to see you'll have yours at home soon enough