This is a Fig8 Puffer?
Their teeth are not nearly as fast growing as SAPs, so im assuming you havent been feeding hard shelled foods often enough. Keep a real close eye on him when feeding make sure he can close his mouth etc. Feed Snails, Prawns in shells etc...as the main part of their diet.
Might i suggest a large plastic tub with an airstone and ramshorns snails, ive been breeding mine for a while, its cheap as chips, throw in nasty fish flake food and replace 80% of the water every 1-2 weeks or so and they look after them selves....I must have about 20 blobs of babies...its a continuing source of hard foods.
Edit
Im not sure but i think id have left him in the tub till he was stable in the water etc...but fingers crossed.