My son has a golf ball size lump under his chin?

Lymph node or cyst. Either an inflamed or infected lymph node or possibly something called a thyroglossal duct cyst. Either way he needs an evaluation with your pediatrician and probably an ENT doctor.
See doctor. Likely you need to see a doctor. The lump could be a lymph node if he has had a recent infection. There are other things that can cause swelling in the neck area -- see your doctor for evaluation.