What the doctors came up with is, yes something's there, but they're going to wait and see what it looks like in 6 months. I am getting a second opinion.
As for my son, first I apologized to him for not telling him right away what was happening. Then I explained that I waited because I didn't want him to worry before I knew what was going on. I avoided all triggering words. Kind of a cop out, but the kid has a clue, and I didn't want to terrorize him. I told him I had something like a blister inside me, and it's not normally supposed to be there but it is, and my doctor will keep checking on it to see if it's okay or it it needs some treatment to make it go away in the future.
He was super concerned, but didn't freak out or panic. And now that he understands why I'm a bit on edge, he's been actively trying to be more accommodating for me. He's a good boy.