Please give us some more details regarding why you can no longer care for your turtle.
A turtle is a very long term commitment that many people do not consider before getting into. I've seen various studies show they can live any where between 25-30 years and others that say they can live upwards of 50 years. Although I don't know which is more accurate, it does show that one must be prepared to care for these wonderful creatures for many decades.
What ever you decide, do not release him into the wild. This is never a good alternative and is very frowned upon. It can kill a captive turtle that can not adjust to living and caring for themselves, and it also can affect the natural wildlife in the area by introducing a foreign species into their habitat.
If you must get rid of him, find an adoptive home for him that can provide the appropriate care he needs... or find a turtle rescue center to take him.