I wouldn’t worry he probably isn’t ready for food even thou ur doc said start him on solids try jus give him a spoon of baby rice to start with rather than a bowl of food,
It will take him some time I started my daughter at 14 weeks as she was ready for solids but i started gradually even now she’s 18 weeks old I
Still give her very little solids,
Don’t panic jus rest it for a few days then try again, what time r u feeding him and are u mixing with milk o water