advice please!

My 9 month old has recently stopped wanting to drink his formula, everytime I offer he refuses. So I started feeding him solids because I didn't want him to starve and I wasn't going to force feed him his milk. Has any other momma had this problem and found a solution? His doctor his concerned that he's not drinking milk but didn't give me any advice. Should I mix some flavor in it ? Any advice will mean a lot. I'm a first time mom. TIA