You do not have to adjust your HSA contributions if your spouse goes on Medicare, as long as your only coverage is the family HDHP. If you turn 65 in Feb 2024, then your effective Medicare enrollment date will be Feb 1, 2024 if you enroll any time between 3 months before and 6 months after your 65th birthday. That means that while you can contribute the maximum amount for 2023 ($7750 plus $1000 catch-up), you will only be eligible for one month of 2024 and your contribution limit will be $775 ($8300÷12 plus $1000 catch-up÷12). (**special rule, if your birthday happens to be February 1, your Medicare enrollment will be effective January 1 and your HSA contribution limit for 2024 will be zero.)
Separately, your spouse may make contributions to an HSA in his name as long as he is covered by a qualifying HDHP, even if he is not the named policyholder. Your combined maximum of $7750 can be divided any way you want, but each of your $1000 catch-up contributions can only be contributed to an individual account. In other words, even if your husband does not currently have an HSA, he could open one at a private bank and contribute his catch-up contribution (up to $1000) under his name, even if you are maxed out on your account.
However, no matter when your husband enrolls in Medicare, if it is more than 6 months after his 65th birthday, his enrollment will be backdated 6 months. That means your husbands enrollment will be effective November 1, 2023. And that in turn, means that if you maximize contributions to your HSA, your husband could contribute $833 to an HSA in his own name for 2023 and nothing in 2024 ($1000 catch-up ÷ 12 x 10 months).
