Dog weight question.????¿?

my dog is always between 11 and 13 pounds. He is some sort of maltease mix or havanese mix. I was told it is healthy when you can feel his bones by the vet who then went on to show me how under the bone was muscle making him healthy. But I feel he is too skinny and would like him to gain a couple pounds and stay at that weight and not go back and forth. I walk him the same 30 mins a day play with him ball for 30 minutes a day and play with him and his toys all day which is constantly wants thrown for him and I don't change his feeding schedule he eats the same amount everyday and no table scraps. Sometimes he gets cut up boiled potato once a week not everyday. But I just really want him to stay at one weight. How can I make this happen without cutting back his exercise or highering how much food he eats daily. I would like to feel some fat on him a little but because it makes me feel like that the dog is okay not just feeling bones and muscles. Help please.