All in all, if you know where he is it's much safer to let the cops handle it. So what if you do sort it out with a few of your "larger friends"? He's probably got larger friends too, and he knows where you live.
The legality depends entirely on the location of the stolen property and the method of recovery. It's not the actual reclaiming that is illegal. We are talking about this particular iPad, which we only know to be on the thief's person, in a house, or in a car. The likelihood of reclaiming it without breaking a serious law is very low.
