H.F.
It is usually between 4-6 months. However, you probably want to check with your doctor. If there are allergies in the family, doctors tend to want you to wait a little longer as it seems to make a difference in whether or not they develop food allergies.
In the meantime, try to increase the amount you give your son at each feeding. If you have been giving 4 ounces try to up it to 6. If 6, then up it to 8. This may help him go longer between feedings. That is what my doctor suggested for us. My son (almost 4 months) now sleeps through the night.
Good luck and hang in there.