How to help smokey gain weight?

So my new ferret smokey didn't come from a very good place. He wasn't fed ferret food, im also not sure he was fed all the time. He's playing and sleeping and drinking regularily but he picks at his food in little bits and he feels really light. He is going to the vet tomorrow, but what can i do to help him?

Never mind this post. The vet said he is a great and healthy weight at 1.02kg. And that its okay that he is a creep feedeelr. His body is just adjusting to a good diet.

He will start gaining weight.  Once, he out and running around. and playing...he will start eating more.