There is not really much of anything you can keep with him. There are some options, but most greatly depend on tank size. Other turtles would be a good option. There are an assortment of fish you can keep with him. Cichlids are a good option, type will depend on tank size. Fast swimmers like danios and silver dollars would also be a good option. I have had relatively good luck with things liek crayfish. As long as they have a safe retreat they can do quite well with aquatic turtles. They are also a good option because they will help keep things clean. Things like you are thinking of would require very large setups. Even with semi-aquatic snakes like garters, the setups for one and the other are quite different. If I may ask, why do you want something besides the turtle? They are quite interesting and I can't imagine much else being more active or responsive to the keeper, besides endotherms.
He is an eastern painted turtle?
What size tank do you have for him?
He is an eastern painted turtle?
What size tank do you have for him?