9 month old not eating well

by Worried Mom
(South Africa)

When my baby was born, he weighed 3.9kgs. I exclusively breastfed my baby 'till a couple of weeks after he turned 9 months. He preferred the bottle more during the day and night and weaned himself. I started giving him formula during the day as he was not gaining enough weight. This was done simultaneously with introducing solids. From 1 meal to 3 meals a day. Formula in between.
He only weighs 7.6kg now - still haven't gained twice his birth weight. Everyone that sees him remarks that he is a "tiny" baby. He doesn't have "baby fat". He will sometimes finish his pap in the morning (only 2 desert spoons full), sometimes eat only half a fruit purity at mid-day and nothing at night. Sometimes he will eat vegetable purity at night, but then only a quarter of a purity stage 2 bottle.
We have tried everything. From giving him the spoon to play with and trying to feed himself, to introducing maybe more solid food as he may not want the "mush" anymore....... Nothing seems to work.
He seems to "gag" a lot when eating, as if the food is making him nauseated, and then he just refuses to eat anything....
He does drink a lot of formula though and is meeting all of his milestones. He seems like a happy and healthy baby and is sitting, crawling, etc.
I have not managed to "solve" the problem yet. Have tried to search any similar stories on internet, but have found none.
Any suggestions?? I am really worried as I don't want to leave this and it may become more serious.
Thanx