I took home Lockjaw at 7 weeks. They are still transitioning from their mothers antibodies/immunity to their own. Plus the added stress of being away from their litter mates and into a new home can sometimes be very stressful for them- did you change the kibble he's on? If so, that could play a part too.
Have you taken him for his initial vet check yet?
The same thing happened to us with Lockjaw. When I took him to the vet for his check up - he had a little bacteria in his stool. Vet gave him another de wormer (just in case) and also a probiotic powder to put in his food. For the life of me I can't remember the name of it. It was made by Purina though. It did get better. You can also try just giving him plain hamburger/rice as well until it starts to harden. Give him a little while to adjust - and never forgot lots of love never hurts!