3 month old, good head control, trys to stand on his legs when i try to put him down, can i let him stand with support or should i keep stopping him?

Let him stand: As long as he is supported by you, there shouldn't be a problem.

Standing: I would discourage him from standing at three months of age because of undue stress thatis being put on his back muscles.

Prop reflex: A 3 mo will extend his/her legs to prop as a standard reflex, it is not an effort to stand. This reflex disappears between 6-9 mo as kids nervous system prepares for voluntary standing. You will not make him bow legged by doing this, but it is more a parlor trick at this age than anything.There are more appropriate things to do with a 3 mo.