all i can say is if you took him from outside you better do a darn good job making sure he has exactly what he/she needs. you've taken it upon yourself to disrupt his life for your pleasure. sounds selfish to me.
so.. you now have a huge responsibility.. a 13 in long and approx 30 year responsibility. think of where your going to be and what you want to do in a few years.
are you going away to college or university? your turt will be the size of a bacon cheesburger by then. and you'll need a 50+ gallon tank. will you be living in a dorm? rooming? or at home.
If you have to move away for school, i would suggest you put him back. unless you plan on lugging all your turtle gear away to school with you.
most dorms wont let you house anything in a 50 gallon tank..
If you love this turtle that much, you'll put it back and let it enjoy the life it was meant to have.
wait ten years, when you have a home, money and a steady full-time job or career.. then you can get all the turtles you want.
why make the little guy suffer because you "want him"?