You should let him go free. Turtles sometimes carry disease too. Peace.

 Let him go!  lol we did the same thing and my son was in love with it.  But often painted turtles will rather starve than eat in captivity so after a few days we had to let him go.

Plus right now a lot of places , painted turtles are getting scarce so any more in the wild will be even more chances for breeding and keeping these turtles around!  In some states (like Indiana) it is actually illegal to keep some turtles!

I know it's expensive, but go to the pet shop.  We ended up getting a greek tortoise and he's wonderful.  Unlike the wild one, he lets you pet him and eats out of your hand.  It cost more, but definitely will feel much better about it being a pet, health issues (like disease) and conservation.

 I think you should let it go as long as it is native to the area you got it from it was born in the wild and should stay in the wild it lives longer that way i think the best way to find out is to ask your vet and they can tell you the best action to take with it