baby needs to gain

I posted about sleep, but after thinking, decided it is more important to get advice on weight gain for my son.
He is 6 1/2 mos and weighs 11 lbs.7oz. 25 1/2 inches long. breastfed baby. eats sweet potatoes 1 time a day. If I pump out milk he takes only 2-3 oz at a time. I offer the breast almost hourly. if not every 2 hours. any advice would be appreciated.

my son doesn't sleep well at night. I nurse him down while gently walking around the room. As soon as I try to lay him down he wakes anywhere from 5-20 minutes later, screaming like he has been abandoned. His cries can be pitiful at times if I don't respond within a minute or two. He is a blond hair blue eyed little boy who is thin. The doctor says he is even a little underweight. Despite all of my efforts to bulk him up. He loves to eat real food, but I only give him sweet potatoes because when he had other things like pears he screamed all night long like he was in pain. He is exclusively breastfed. I don't eat any dairy as it seems to aggravate the situation more. During the day my boy is pretty happy. He doesn't like me to sit still for long periods. He likes to walk around outside, and see all of the new things. As for strangers, he will smile and laugh at them as long as mom is holding him. He will only go to dad for 45 minutes before he is crying for mom again. His stools are unpredictable. He for sure poos once week sometimes a more often. The stools are thicker to start almost clay like and then end with liquid.