After 3 months, I would think the nightly escape attempts should have ceased. Sounds like something is keeping the snake stressed out. Are you handling him a lot, or daily?
Krazy has the right idea. If you don't have foliage, you could wad up some newspaper and toss it about the tank, until the snake feels more secure. Also, try not handling at all for a few days before the next feeding attempt.

If that doesn't work, I would suggest getting a fecal done, maybe he has internal parasites.