My DD is a smidgen above the 10th percentile and was within the 25th from birth up until she turned one. I have spoken to numerous doctors and even spoke to a paediatrician who said she is fine, shes very active and has a petite frame. I haven't been told anything even when i mentioned her not eating alot. They said shes a kid and she will sort herself out.

In all honesty i would hold off doing anything until you speak with your paediatrician. I don't agree with the oil not at his age. To me what he is eating sounds fine and if he is happy and healthy thats all that should matter.