I would rule out any medical problems since you mentioned it happens when he is picked up sometimes too. Has he been checked for worms or any other medical problems?

Ruling out medical issues, take a lesson from Ginger.... when Bob gets too rough she lets him know it. This is only a 10 week pup we are talking about so it shouldn't be that hard to nip this in the bud now (no pun intended).

BTW... Bob is really a cutie!