My 4 months old vomits right after nursing I ve done everything and am really worried. He weighs 7.30kg which I think is small considering his feeding?

VOMITING. Baby occasionaly vomits after feeding either due to overfeeding or not burped properly. They gain 2 lbs. Every month until 3 months and1 lbs monthly thereafter. Doubles birth wt. In 6 months. Your baby looks fine.
Could be.... Either overfed, reacting to something you are eating or he has gastroesophageal reflux.Your pediatrician should be able to help.If he is gaining weight and not overly fussy, not an emergency.