Hi Eliz!! Try to BF him at least 4 times per day. If you are already doing this, then add one more BF. Is your lo having solids? If he is, you can also add some yogurt or cheese to his diet. Just remember that solids before 1 yo are just for fun, so milk should be the main source of nutrition for your lo until he is 1.
Could you post your feeding routine so I can take a look and help you out to get in your lo more calories?