Our grandson is 20 months old. He was born 4 week early. Ever since he started to stand he has been on his tiptoes. He hasn't started walking on his own yet. He starting cruising at a normal age, when he cruises he very efficient at it, he also will walk on his knees at times. When we holds his hand to help him walk, he may take a few steps (on his tiptoes) then sit down. We have tried different type of shoes such as squeakers, high tops, etc. What would be the appropriate type of doctor to see for this?