When you do release him, it could be best to make a point to do this as far away from people and houses etc. as you can manage. It is possible that his exposure to humans (and their scent) will mean that he fears humans less than a normal wild squirrel would. One manifestation of this situation in an adult squirrel could be outright aggression toward people (even though that's not what you would expect, as the wild animals' minds don't make sense in this regard). Another could be letting humans get too close to them. I can imagine a park where there is some jackass who stomps or kicks him for the fun of it.