Congratulations. He probably just needs to adjust to his new home.
Try some boiled chicken, white meat only and some veggies. Greenbeans
peas/carrots and mix those in with his food. No fat and very healthy.
I use the frozen veggies. I let the veggies defrost over night in the refrig
and alternate every two days in the variety. Easy and healthy. About
1tsp each with each feeding. Good luck