You have every right to feel the way you do. If you're uncomfortable with what happened, you're uncomfortable. Just wanted to say that.
However, it's one of those things where life just doesn't go according to plan sometimes. This boy just doesn't get some social cues, and other people may indulge him (sharing the peace when it's not technically the time for that), but it doesn't make you bad for not wanting to do it right then.
Possibly one thing to do would be to just kind of pat his hand as you push it back toward him, and say quietly "Not right now, sweetie. I'll shake your hand later." when he first tried to grab you.
Another thing that you can do, more proactively, is ask the priest how you could handle it. Presumably, the priest knows the young man. Explain that it's not that you don't want to share the peace with him, it's that going up to receive communion is a solemn time for you and you don't wish to interrupt it right THEN to shake the young man's hand. What would be the best way to handle this in the future, so that he doesn't feel rebuffed and you don't have to interrupt your prayerful time?