Biting puppy scares my son.?

We have a new puppy. He is an 8 week old Border Collie named Marvin. The problem is he is obviously teething and biting /nipping everything and the family. My 4 and 1/2 year old son is petrified of him.He nipped him yesterday and that scared him so much he now screams if the pup goes near him. He jumps on to the back of the sofa to get away from the pup and this makes the pup yelp and my son which doesn't help. I have tried explaining to my son but he doesn't understand, he is just scared. Any tips woyld be gratefully accepted. Thanks.