Im raising that flop every single time. His overbet usually means he has a very weak holding or a very strong holding. Basically he could have either a low kicker K, a PP lower then a K, or middle pair... either that or he has 2 pair or lower set here. If he has a weak holding, your not gonna make any more money off him anyways, unless he hits a higher set or improves to 2 pair. If he has 2pair or lower set, you want him in now.

Also, there is a possible straight draw, someone could be holding 35. In which case I wouldn't want someone holding that coming along for the ride. Cuz if a A or 6 hits on the turn, it will make my hand ALOT harder to play.