He will need to create a new separate account with a new User ID if he is going to use the online editions and file his tax return as Married Filing Separately.
If you are filing married filing separately---usually the worst way to file----you are preparing TWO tax returns---one for each spouse. And you have strict rules to follow.
If I am filing a separate return why do I have to list my spouse’s information on my return?
Even if you file separate returns (the worst way to file) you each have to list each other's SSN's and some other information on your own tax return. The IRS can then cross check to make sure you are not "double dipping" for itemized deductions, dependents, etc.
If you are in a community property state, there is more information that will be needed.
Community property states: AZ, CA, ID, LA, NV, NM, TX, WA, WI
https://ttlc.intuit.com/questions/1901162-married-filing-separately-in-community-property-states
https://turbotax.intuit.com/tax-tips/marriage/five-tax-tips-for-community-property-states/L4jG7cq7Z
If you were legally married at the end of 2023 your filing choices are married filing jointly or married filing separately.
Married Filing Jointly is usually better, even if one spouse had little or no income. When you file a joint return, you and your spouse will get the married filing jointly standard deduction of $27,700 (+$1500 for each spouse 65 or older) You are eligible for more credits including education credits, earned income credit, child and dependent care credit, and a larger income limit to receive the child tax credit.
If you choose to file married filing separately, both spouses have to file the same way—either you both itemize or you both use standard deduction. Your tax rate will be higher than on a joint return.
Some of the special rules for filing separately include: you cannot get earned income credit, education credits, adoption credits, or deductions for student loan interest. A higher percent of your Social Security benefits may be taxable. Your limit for SALT (state and local taxes and sales tax) will be only $5000 per spouse. In many cases you will not be able to take the child and dependent care credit. The amount you can contribute to a retirement account will be affected. If you live in a community property state, you will be required to provide additional information regarding your spouse’s income. ( Community property states: AZ, CA, ID, LA, NV, NM, TX, WA, WI)
If you are using online TurboTax to prepare your returns, you will need to prepare two separate returns and pay twice since with online, you get one return per fee.
https://ttlc.intuit.com/questions/1894449-married-filing-jointly-vs-married-filing-separately
https://ttlc.intuit.com/questions/1901162-married-filing-separately-in-community-property-states
If you are filing Married filing Separate, your spouse files his own return. You will need to enter his personal information (name and SSN) on your return, but you don't include any of his tax data. You said you have to file separately, but if you were married as of 12/31/23, you can file jointly with your spouse, which in most cases is a much better choice. MFJ vs. MFS. Please review this article.