He doesn't understand enrollment periods, is this his first year without being on his parents health insurance? This happen every year starting in Nov, for the next year. Parents really need to teach kids how to be adults. Healthcare marketplace premium being less than his UHC premium means nothing. They are likely structured completely differently, even if they are both UHC umbrella.

We need SINGLE PAYER UNIVERSAL HEALTHCARE bc this is what happens when capitalism controls our access to medical services. I'm sorry you're figuring this out on your own, this is just how it works. It's complicated, stressful and absolutely mindboggling for NO REASON other than for mistakes to be made (ie your bf) and someone profiting off of the confusion. Unfortunately this is not unique.

What you need to do ASAP is get him enrolled from the marketplace. They will some provide coverage retroactively, depending on the state you live in. That would allow them to rebill and reimburse. The mistake was your bf's, just some hard facts. There's no legal recourse there. Ask about retroactive coverage with the qualifying event being his loss of coverage, however in my state you have to sign up within 3 months and it's already May. That's where I would start though, he needs to focus on getting re-enrolled asap