[anonymous]

You need to see a vet asap.

First of all it is recommended for the pup to stay with the mom and sibs till at least 8 to 10 weeks. Is it possible for the pup to return to the mom for at least 3 more weeks?

The diet is wrong. NO GRAIN-FREE. Soft food would be best. What was he being fed before? No changes for a month is usually recommended. Does he even know how to eat?
TALK TO A VET, in real life not on the internet, get him examined. Has he been wormed?

Best of luck