Unless your spouse has family HDHP, he will be eligible to contribute only up to the limit for self-only HDHP coverage. If your spouse has family HDHP coverage on December 1, 2018, your spouse will be eligible to contribute the full-year family limit based on the last-month rule, but must then maintain that coverage throughout 2019 to avoid a 10% additional tax for failure to remain an eligible individual throughout the testing period.
