He is too young to understand that it's naughty, and it is normal. BUT you do need to teach him that it's not ok, or he won't stop doing it later. I would hold his hand when he's done it and say no in a firm voice. I would also explain that it hurts so it's not nice to pinch other people as we don't want to hurt them. Also try not to say "ow" when he does it (easier said than done I know) so that it doesn't encourage him.