Grandson not growing. Help

My 13 month grandson is not growing. He has only recently doubled his birth weight and is on the bottom percentile. He is meeting all of his developmental milestones, but has had numerous visits to A&E with high temperatures and breathing difficulties, always diagnosed as viral. A&E doctors have commented on his weight and on the last visit forwarded details to the paediatrician who has been monitoring him which has resulted in a phone appointment. My grandson eats everything and is still being breastfed. Understandably my daughter and hubby are really concerned as am I and don't know what else they can do. Has anyone out there encountered a similar problem?