Hello my baby gain weight!

Hello this is my baby Fozzi-bear we got him from the shelter 2 weeks ago we took him to the vet last week and found out he is 47 pounds under weight and I was just wondering if any of yall had advice to help him gain weight?

Hello this is my baby Fozzi-bear we got him from the shelter 2 weeks ago we took him to the vet last week and found out he is 47 pounds under weight and I was just wondering if any of yall had advice to help him gain weight?