Refusing bottle

My son has been refusing bottles the past week. He is already on the smaller side (sga) barely drinks 2oz per feeding and is now refusing to drink milk. I BF and FF him ( pls before you judge I've tried very hard to BF him exclusively, but failed. We've been to 5 different breastfeeding clinics to no avail so I have to supplement with formula) he has been spitting up a lot every feed. And when I take him to the doctor they always say well if he's putting on weight he's fine. But I can see him in pain as soon as he's done drinking he seems uncomfortable pushes up his body cries and then takes half of it out. He also burps all day (wet burps) and off lately he is refusing bottles as well. Please help he is 4 months old I really feel so bad for him.