Thank you for your question and welcome to Just Answer.
Your son has a tooth abscess on the front tooth nearest the bump. This should be a baby tooth at this age and the abscess is a result of either some decay or from a traumatic blow to the tooth at some earlier time. He needs to be seen by a dentist in the next day or two to have this situation evaluated. An X-ray should be taken to determine if the tooth needs to have a baby tooth root canal performed or removed. The permanent front tooth that will eventually replace this baby tooth normally erupts at age 6 years and it might be time to just get the baby tooth out of the way to make room for the permanent one, which might be close to the surface anyway. In the meantime you can keep him comfortable with children's Tylenol or comparable medication. The dentist might want to place him on antibiotics before treating the tooth so a call to the office today is a good idea so a prescription an be called in for him.
I hope this answers your question. If you would like to discuss this further or have any additional questions, please reply to my answer and I will get right back to you.
Sincerely, XXXXX XXXXX DDS