Need help about diet!

My son is 15 months old now. And he hates solids except his snacks(that too only few bites at a time) but his drinks 7-8oz per bottle for atleast 5 times a day which ranges about 40oz (in which i add 2-3tbsp of his gerber cereal which he wont eat in a bowl as solid) and 4oz of water. I really want to transition him towards a bit more solid food.

What else can i do to increase his solid amount?

His doctor isnt concerened about his growth but i am because i feel that what if he never gets the hang of eating normal solid food like us??

Please help!