My 4 1/2 baby has been vomittimg about 30 mins after feed the gagging/choking & vomiitting curdle milk?

Last wk I got him weighed & he had dropped from the 50th percentile to under the 25th in over a month. So the HV suggested weaning him & giving more formula (as he is fed/formula fed). So I was giving more formula during day & then feeding him at night. He was fine for a wk. Then all of a sudden he started projectile vommiting after b/feed & producing the curdle milk & also after formula. For 3 days he kept doing this. Took him to Drs & they said it's viral infection. Since thurs afternoon he had stopped vommiting. I didn't give him anymore formula after tues afternoon vomit. Yest I gave him 3 bottles of formula 2-3 oz in each. This morning about 20 mins after b/feed he vommited again same way. He also has a cold & seems chesty. Could this be intolerance/allergy/reflux can this stuff come up this late he is 4 1/2 months would this not show up earlier? Could it be due to the larger quantity of formula I gave him that wk? Can anyone pls help I'm really worried I don't want him to lose weight.