I've been through this a couple of times...

A nice warm (not hot) soak tends to help them relax their bowels. Also, depending on how long it's been going on, you could try adding a bit of mineral oil to his food (I've actually had to shoot straight mineral oil in a syringe down a tegu's throat--but that was after a LONG constipation time).

One thing I've noticed about PREVENTING constipation is adding a bit of cod liver oil to their food seems to really help.

How long has it been going on?