Baby still fussy with drinking 😞

Took my son to express care when I got out of work yesterday 3:30pm because he had not drank a bottle since 5am, doctors gave a few reasons why he maybe won’t drink (try new bottles or sippy cup, maybe just wants solids, maybe just too distracted) she said he looked fine no signs of dehydration or anything. I went out and purchased a sippy cup and nothing, tried his tommee tippee bottles he had before I switched to avent and nothing) he didn’t drink formula til 8:00pm. He has a bottle again this morning at am with no problem and he is refusing again for my mother (I’m working right now again) I’m getting so frustrated because I know it’s important he gets his formula. Any advice? Anyone else go through this? He has a physical on Tuesday so I’ll bring it up again.