1. Your husband must stop contributions when he has "other" coverage, including medicare. If he enrolls during his normal enrollment window (65th birthday +/- 3 months) his enrollment will be effective the first day of the month he turns 65. Since that is December, he can contribute 11/12ths of his annual limit. If it is a family HDHP, and the limit for 2025 is $8550, then his limit for 2025 will be (11÷12) x ($8550 plus $1000 catch-up) = $8754.
2. Once funds are contributed to an HSA, they can be spent at any time for qualified medical expenses, even if the person is no longer eligible to contribute. Your husband keeps his HSA and can use it to pay for qualifying medical for himself, his spouse, and any tax dependents, even if is not eligible to contribute.
3. HSAs are individual accounts. You never "transfer" accounts and they are never joint. Your husband owns his own account. You can open an HSA in your own name, and contribute if you are eligible, but you can't contribute to his HSA.
