Yes. Even though you had a final judgment in December 2024, the IRS considers you married for the entire year if your divorce wasn't officially finalized on or before December 31.
You can amend your return to Married Filing Jointly. This will combine your two accounts in the IRS system. This should satisfy the balance the IRS is asking him for. However, since your current Single refund is still processing, you should wait until you received the refund before filing the amendment (Form 1040-X).
Since your husband has a balance due notice with a deadline, he should call the IRS at 800-829-1040 (or the number on the top right of his notice). He should explain the situation, that you were legally married on Dec 31, 2024, and that he is waiting for your original refund to clear so you can both file a joint amendment. He can request a 60-day or 90-day collection hold on his account, which will stop the IRS from sending further notices while you wait for your refund to arrive.
