Retching and no weight gain

Hi, My son is now 19months old, he had a bad stomach bug when he was around 9months old, which took him months to recover from. He weighed 21lb in January and still weighs the same now. He barley eats anything and from the minute he wakes up in a morning, he starts with retching episodes. He tends to start as soon as he sits up in a morning for an hour or two (but he does pick at breakfast and has milk during this) then he can do this again, sometimes five or six times in a day, sometimes lasting seconds other times it drags out over an hour or two. He's rarely sick with it. But i don't understand what is causing it. Is this something i should be worried about? How can i get him to eat more? Certain foods cause him to retch at the sight of them. On occasions i have had to go take care of him during the night because he's baulking in his sleep. What could a possible cause be?

Thankyou