Concerned Mom

I just had a quick-ish question: My son gets these weird spots. They look like bug bites in the beginning (red, swollen with a whitish bump in the middle) but these bumps don't seem to irritate my 2 year old at all. Then after a few hours the spot is no longer swollen but it turns into a brownish slightly raised bump that never goes away. He has 10+ of them and some of them have been there for a year and a half. They occasionally re-swell, bit again don't seem to irritate my son at all. I've been to doctors and dermatologists and no one knows what it could be. Any ideas?