I agree with the others. The kitten needs to be seen by a vet ASAP. Growing babies need to be able to keep food down to grow up happy and healthy.
(I missed the not keeping food down... ;( )
I agree with babygirlhawks. I would take this kitten back to your vet for further help. Tell the vet that the kitten is not keeping any meals down.
If the kitten is stumbling etc. there must be a reason. This kitty needs more help from the vet. Seven weeks is of course less than 2 months. Eve
Take him to a vet, the kitten might need some antibiotics or further help
Weaned a little young in my experience, so hopefully kitty will grow out of this. Give him a little while longer and keep up the care.
Ask your vet for advice, too. 7 weeks is still a baby and he has a bunch of growing to do.