Hello and thanks for the question. This might be a simple skin irritation (from something like dry skin) or it could be a breakout of something like a cold sore. Cold sores have a blister look to them and they will form around the mouth, nose, and facial area. Normally, the cold sores will itch, and be painful if they become very large, but if it is a small area, it might not be noticed. These are fairly common and can be treated with over the counter Abreva or (depending on his age), prescription medications. You can simply watch it for a few days and see if it begins to worsen. If it does, he will most likely need to have his Dr take a look at it and give him something to clear it up pretty quickly.
Abreva is for children 12 and up. So, no, I would not use it. You can simply watch the area for a few more days and then call his Dr if not any better. In the meantime, you can use Carmex on the area. It is often effective on conditions like this.
Your are very welcome, hope this clears up soon!