Changing filing status from MFS to MFJ

Married in 2024

Husband never has enough taxes withhold so I filed in March as MFS and got my refund.

Husband filed extension and mailed payment he thought was due

Husband just realized he has another 1099-B showing massive capital gain.

If he files MFS - Owes another $30K

If he files MFJ - owes $19K

He has not filed his return yet. What is best way to handle this?  Do I just file an amend  and change status to MFJ and include all his income? Will they be able to account for his payment made with Extension?

Thanks in advance.