I was a bit confused by the OP’s initial comment: “Premium is increasing from $220 a month to $350 a month…and the letter says that includes his current subsidy.” But then I realized what was going on.
The ACA premium subsidies are designed to bring down the cost of insurance to an amount the subsidized person can afford. For a person who makes $X/year, the subsidy is whatever will bring the cost of their insurance down to $Y, where $Y doesn’t change from year to year.
But there is a caveat: the subsidy assumes that the person buys the reference insurance policy, the second cheapest Silver plan. If a subsidized person bought the second cheapest Silver plan last year, and buys the second cheapest Silver plan this year, they will pay the same, by law. Their subsidy, no matter which plan they buy, is the amount of money that will make the second cheapest Silver plan cost $Y. If the plan they buy went up more in cost this year than the second cheapest Silver plan, they will have to pay the extra.
So, an example: Ava is entitled to subsidies. Because of her salary, she is entitled to spend $200/month for health insurance, no matter how expensive her insurance would be if she didn’t have subsidies. In 2016, the second cheapest Silver plan, Bob’s Insurance, cost $500. She gets a $300 subsidy, which she decides to apply to Acme Health’s $600 plan. In 2016, she pays $300/month for Acme, even though she could have paid $200 for Bob’s.
In 2017, she hasn’t gotten a raise, and she is still entitled to pay $200/month for insurance. This year, the second cheapest plan is BeWell Insurance, which costs $550/month. She is entitled to a $350/month subsidy. Acme Insurance costs $750. If she chooses to stay with Acme, now she has to pay $400, versus $300 last year.
But the point is, in both years, she can pay $200/month if she buys the second cheapest Silver plan. She choose to go for the more expensive plan, and the taxpayers aren’t going to subsidize her for going for something more expensive when she could have picked something less expensive. We’re subsidizing hamburger, not steak.
For people who are not subsidized, the calculations are completely different. After several years of lower increases, many of us are seeing big increases this year.