My Four Year Old Son Has What Appears Appear To

My four year old son has what appears appear to be a blood blister on his neck about 1,5-2 cm in diameter. My go attempted to put liquid nitrogen on it a couple of weeks ago, and really didn't do a complete treatment and while there is some blackening a=occurring on the tip my son is very self conscious about it, and it may hurt a little rubbing on shirts etc.
Any thoughts?