What's his weight? And what was his weight 3 months ago? Deborah suggested he might be trying to "catch up" his length to his girth. If he's been off food for a couple of months, but still looks like a healthy body style, then I wouldn't worry about it too much. If he's thinning out, but only losing a little weight, I also wouldn't worry about it too much. If he's looking thin, and borderline emaciated, then it's time to do something.

If F/T hasn't been working for you, then go to a pet shop, buy a small rat, show the snake the rat through the enclosure and try to get him interested in it. Then, kill the rodent right before you put it into the enclosure. Bonus points if you kill it completely, but get it in the tank while it's legs are still twitching. Attempt at night, and if possible, position yourself such that you can watch him, but he can't really see you... You want him focused on the small prey item, not the giant potential predator (you) looming over him. I have a decent sized piece of cardboard that I put between myself and the snake on occasion, and I can peer over the top and spy on him, while he can really only see the rodent, and the solid cardboard background.