hello. welcome to HCM Forum.
At your sons age, it is quite common for babies to spit milk. It is not serious as his wait gain is normal. make sure to check it as recommended by your pediatrician
1. It is less common for breast fed babies to spit milk when compared to top-fed ones
2. The milk must be curdled and not fresh milk. (off and on exceptions are acceptable)
3. It is uncommon for breast fed babies to overfeed when compared to formula fed babies
Now it is very essential to take the child on shoulder after every feed and make him 'burp'/ break air. After this make him lie down for say 7-8 minutes and then pick him up for re-burping. You will help minimize vomiting
pains and improve digestion in the child.