Nice and what a very generous uncle you are. He may not be aware of what bitcoin is today but after the right time, he'll value it more than what he has soon.
P.S. To avoid losing this btc, you should create kinda back-up procedure.
Yes, although you have given it to your nephew still create a back up for yourself so if ever it was accidentally lost you can recover it easily with your back up.
Execute what you are thinking, everyone's agreeing with you.
This idea is so great and just shows how loving and generous you are to your nephew. But looking at the big picture I don't think that it will be possible now because you just said that your nephew is just 4 yrs old. Maybe the better thing that you can do is to make that amount grow and when times comes that your nephew is old enough then you can already give it to him.