What Saskia said. I recently took on a mare that was primarily used as a lesson horse, but wasn't ridden regularly, and our first week or so together was tough getting her going! Now just a month later, she's a different horse. I ride 5-6 days a week, but I keep a rotation going between the covered arena, dressage arena, and an open field for 45 minutes to an hour each ride, and she seems to enjoy it now. Whereas our first week together I had to work hard to get her forward, I'm now having to keep slowing her down.
I would suggest that you keep doing things that change up the routine a bit. Try to keep his mind working and engaged and perhaps he will start to enjoy it again.