Maybe change the type of hay given? My horses do just fine with grass hay in the summer, but sometimes people drop my alfalfa or grass/alfalfa mix bales by accident and I'm forced to use them. Once my horses get a taste of the alfalfa, it's like they're addicts. They get mopey about eating anything else for a while, and will leave large amounts of (nice) grass hay in their stall. Then they eventually forget about it, and eat every last scrap of their hay.
Perhaps you need to check out the hay? See what's in it, see if the type has changed, or if the provider (i.e., where they got the hay from) has changed. I've also known some barns to cut way back on hay during the summer, as the horses have access to pasture. But if your horse is free to choose between stall and pasture, he may be choosing the stall more frequently, and not get out to eat due to heat and bugs.