I just want to clarify here: if your son has Medicare, it means it is because he is disabled. You would not want to put him on a private insurance plan. If you meant Medicaid, that is low-cost insurance provided by the state for the indigent.
$900 a month seems high, but is about average for what many people are paying these days. But that amount is merely drops in the the ocean of what you could have to pay if your husband had an emergency that required medical attention. You could be bankrupt within weeks.
And I'm going to disagree with CowboyCO: both my husband and I pay for the family plan at both of our jobs. My son (his stepson) has dual coverage, which means no copays, no deductibles, and everything he needs is covered 100%. Worked out well for us, because he has had to have two bone marrow transplants within the last two years (always healthy before) that would have cost us several thousand dollars each if we had high deductibles and probably would have cost us our house if we didn't have any insurance.
Insurance is a little like gambling -- you are betting that you are going to use more than you spend in premiums. The difference is, everyone needs medical care at some point, and I think kids need it more (think skateboards and sports and trampolines and surfing...see where I'm going with this?). Trust me, if you can't afford insurance, then you absolutely can't afford to be without it.
OP, can DH get a different job that offers insurance? Look around at different individual plans, but I don't think you'd save any money there. I also know plenty of people who work part-time jobs to pay for their health insurance.
Good luck.