You JUST caught him. It's a fact you cannot say "he is doing great"
If you know so much information then why do you need to know if a 50 gallon is big enough? Frankly for a ten inch turtle, its not big enough at all. Try 100 gallon MINIMUM for a turtle that has lived in the wild its entire life. That's a simple fact. Sorry to say.
I would release him IMHO.