Anyone else’s baby have trouble gaining weight?

My son is almost 8 months and weighs 16 pounds. He is literally still in 3-6 month clothing. The only thing slightly chubby about him is his cheeks, he has no rolls! It is SO hard to get him to gain weight. His doctor said he’s in the 10th percentile and wants him to be gaining more weight. She told me to put rice in his formula but that clogged his bottles up and wouldn’t mix right. We have tried baby food, mixing rice into the baby food, and putting rice milk and baby food together so he is getting more calories. We try and feed him every time he even gets a little crabby. Nothing works!! I think it may be because his father only weighs 120 pounds and has always been small. Any ideas on how to get him to gain weight?