I went through this with my son. He put on 2 pounds last month but still looks skinny. He is very tall for his age , so I guess that makes a difference.
My son didn't put on a lot of weight 10-13 months of age. This was the age he had started crawling and cruising and then walking, so he hardly put on any weight. He used to be super active(still is).
These are the fatty foods I give him: full fat milk and yogurt, full fat cheese , egg (almost every other day) , lot of full fat butter , avacadoes , banana , potatoes, other carb rich foods like rice, greek yogurt , brocolli cheese soup etc
It doesn't mean my son eats all these all the time, somedays he just refuses them. But I look over the food he eats over the course of a week and make sure he is eating enough.
There was a time he wouldn't eat anything much for M. (he used to be sick often because he started daycare) and his pediatrician had asked M. to give Pediasure on days he doesn't eat much at all. I gave this for a couple weeks I guess.
Your son is still too young to have all of the above foods I have mentioned. After he is a year old , you can give him peanut butter sandwiches etc