Need Help with supplementing baby

Took my baby EBF baby in for his 6 month check up (2.5 weeks late) and he is measuring in 11th percentile for weight :( doc says I need to increase calories by supplementing with formula after feeds but he will not tolerate the bottle or formula- I've tried open cup method, syringe- nothing. He also will gag at any puree type consistency. He will occasionally chew on more solid foods (rice rusks) but if any of the bits get in his mouth he gags and spits them out. I dont' know how to get these extra calories into him!

(I don't remember the phrase but here is a quick sum up of post): EBF baby needs more calories but won't take bottle or any solids. Looking for creative ideas.