You're on the right track getting him a pond. Aquariums won't do for grown turtles. You can make a setup out of wood, buy some liner, and go from there. Problem is your lighting and location.

I built two temparary indoor ponds out of simple 2"x2" 's and plywood. I have two adult red ears and two adult ybs. You can buy an aquarium elongated filter to fit the width, use a sumberged pump to circulate the water through the filter(attach it to a piece of hose, and afix a heatlamp on the plywood panels on the sides.

This is what I've done and it's worked fine.