Bumps on toddlers chin and neck

My 4 yr old has small bumps on his neck and skin. These resemble a pimple, but aren't. The few times I've gotten something out of them, it feels like a piece of sand. Our doctor said it was a virus caused from swimming, and usually lasts a few months. At the onset of these, he hadn't been swimming for months! Now, almost 9 mo. later, he still has them. We keep him clean, and scrub them good when he's in the tub. They'll look like they are going away, then he'll get a bunch more. Anyone ever seen or heard of anything like this before, and, if so, what can you do to get rid of them? They don't hurt or itch