What is a round growth on your son's gums that has had pus in it and went away and then came back?

It sounds like he could have an impacted tooth and you should take him to the dentist as soon as possible. If you can't afford a dentist, then go to the "free clinic." Usually a dentist will put your son on antibiotics before working on the tooth. Also stress or certain acidic foods can cause this, but leave that up to the dentist to answer. Until you get to see the dentist take tepid bottled water with a good tablespoon of salt and have your son swish this around the sore spot on his gum and repeat this several times a day. Do not poke the sore spot, try to pin prick it to release the puss and only have him GENTLY brush his teeth with a mild toothpaste ( baking soda is great ... take a damp soft toothbrush, dip it in baking soda and brush gently staying away from the infected area.) Good luck Marcy