He's lucky to have found you. I too have tried finding foster home/rescue groups that would take the 4 feral kittens that I trapped last July. Unfortunately these were all full and some didn't even return my calls. I could surrender to the Humane Society(since there weren't any NO Kill shelters in my area), but they will put them down since they were over 2lbs. I couldn't keep them since I have dogs. I had no choice but to release them and provide food and shelter for them. They are happy and healthy and they have a chance at life. You did what you could and if you still couldn't find anyone taking him, it's better to let him go. And if you can, provide food and shelter for him by insulating a 35-45 gallons tote filled with straws or fleece he would come back and stay in your backyard because it's safer there.