At 9 weeks, it is astronomically unlikely it is true aggression. Not impossible, but very, very unlikely. By getting him at 6 weeks, you lost out on a critical window of him learning bite inhibition from his littermates and mother. So, that now falls on you to teach him.
When he grabs, you can either 1) redirect to a toy & keep one handy. If he spits it out and goes for you again, redirect. If it happens again, remove yourself from him by stepping over a gate or closing a door (This method immediately with no toy is method #2). Teeth on skin=no fun. 100% of the time.
Is he resource guarding, is that when he is growling and snarling? Or when he latches on?
If he is getting particularly bitey after about 2 hours or in the evening, he is likely overtired and needs an enforced nap. Think of a cranky toddler if that helps.
How are you picking him up? Where are your hands placed? I'm also wondering if the gastrointestinal infection made handling painful.
Dr. Ian Dunbar could be a helpful source for more information on teaching bite inhibition. Kikopup on YouTube has good videos that handle things like body handling.
