Rash Red Bumps for 1.5 Months

My son has had these little tiny red bumps all over his arms, back, belly, and sides of face/ neck since he was a couple weeks old. They are not raised at all and they go away for a second if you push on them. the doctor first thought it was a viral rash but now that it's lasted a month and a half she has no idea what it is. she said whatever it is it's something to do on the inside because the rash is not raised up on the outside so it has nothing to do with a skin irritation to clothing or detergent or anything like that. Has anyone else ever had a baby that has had these little red bumps rash for this long and figured out what it is?