Hi H.,
They thought my daughter had this, turns out she didn't (it ended up being a cyst underneath her eye). But it's not that rare, it's just not talked about that often mostly because for adults, it's considered a form of an STD (not for kids)
It's easy to catch from other kids on toys, direct contact, or especially if another one of your kids has it and the kids bathe together - that's an easy way to catch it as well.
I don't know where your son's lesions are located, but I read about alot of success for people who used "Liquid bandage" over the lesions. It eventually dried them all out.
There's some good parent information on message boards actually based out of Australia called "well baby" (google molluscum and parent and message board and Australia).
Treatment, you want to keep your other kids from sharing towels, baths, toys, etc. They can be scraped off by a dermatologist, depending where they are at you want to make sure you get the "core" out of the lesion. If you don't, it will come back and if the core touches other skin, the rash will go there as well. Good luck.
Drs that treat is are Dermatologists. The pediatricians usually have a "wait and see" approach to them.
Good Luck!