Moms that formula and breast feed!! I need help!

So my LO is 4 months old and he is both formula and breast fed... lately he’s only been wanting breast and not his bottle. I’m not sure if it’s because he has a cough and it’s congested. But he refuses formula. He’ll MAYBE take 1-2 bottles a day and about 2-3 oz when he used to take 5 bottles a day and 4 - 6 oz a day. I’m really worried he wants breast. Has anyone struggled woth this? And how did you get them to take bottles again??? Also on dec 16 I took him to his pediatrician to get checked because he had a rash on his face and at that time he was weighing 15 lbs and 2 oz. and now at his 4 month check up on Tuesday Jan 7 he was 15 lbs and 10 oz he didn’t gain weight at all!!! Please I need some advice on how I can get him to take his bottle again. He was doing so good with formula and now barely wants it and wants only to be nursed but I don’t have as much milk to fill him up! Im really stressing out about this.