I assume that the $3,183 (or at least your $333 catch-up portion of that) was contributed to your HSA, not his. His contributions must be made to his own HSA. Yes, the last-month rule can be applied to allow your husband eligibility for self-only coverage for May through July. However, I think that you failed to account for the fact that using the last-month rule makes him eligible for the full $1,000 catch-up contribution, not just 8/12 of $1,000. His contribution limit without using the last-month rule would be 5/12 of $4,300 plus 9/12 of $1,000 for a total of $2,542, but with the last month rule his contribution limit is $3,867.
Keep in mind that using the last-month rule requires your husband to remain an eligible individual throughout 2026. If there is any month in 2026 that he is not an eligible individual, the $1,325 contributed for May through July will be subject to ordinary income tax and a 10% additional tax on your 2026 tax return.
