14 months old boy picky eater not gaining weight

What else i can do to make my son gain weight. I try my best to feed him anything that is healthy for him but it seems not to help him to gain weight. The doctor we've seen told me to gave him what i cook at home. My son will eat one or two spoon of food then will spit all the rest of the food that i put in his mouth. The Dr. also suggested of transitioning him from formula to whole milk, which i did. He still not gaining weight. I also putting gerber cereal with the whole milk that he drinks. He is 29 inches tall. I dont know if its in his genes to be thin because i am filipina and his dad is american. i am 5'2 and his dad is 5'11... And i also notice that the only thing getting bigger is his belly.. his arms are thin and body.. He is our first son.. Pls. help im open to any suggestion...

[Renee Radenberg]

Would you please state your son's age so I am able to answer your question properly. Thank you.