He may have just burned the roof of his mouth on food or something. I have that right now.
No way to tell you what it is...I can see it. I do know this...I have seen, more than once, an extra "tooth" come thru a child's roof of the mouth. It's never been lethal, can be cared for by your Pediatrician OR a Pedi DDS. It's not common, but not rare. It also could be something else, so let someone in the medical field check it out soon, as not to crowd or alter your child's incoming teeth.
is it soft or hard it could be a blister , how long has he had it, I would say that it may be a good idea to have it checked out it will put your mind at rest .