You need to escalate this to your divorce attorney who needs to get someone on board from their firm or another firm who is a tax attorney. (Not all attorneys are expert at tax matters.)
You don't give any details, so let me make a few general observations.
1. For 2019, if you were legally married as of 12/31/2019, you can file as married filing separately. You do not need any of your spouse's income or deductions, you only list your own income and deductions, that's why it's called separate filing. You only need your spouse's name and SSN, and if you can't even get that, you can file without it by mail.
2. Married filing separately may not be that simple if you live in a community property state. See a tax professional.
3. If you filed joint returns in past years that were incorrect, because they left off information about your spouse's income or business, then you are jointly and equally liable for any interest and penalties that result from the false information. The IRS can come after you for 100% of the tax and penalties if they want to. You may be able to get some relief but this will need expert help.
4. If you have filed no tax returns over the past years, you can probably help yourself by filing "married filing separately" returns for all those past years.
5. The statute of limitations for tax penalties is normally 3 years, but it is 6 years in the case of fraud or deliberate negligence, and the clock does not start until you file. So if your spouse has not filed a tax return since 2010, the IRS can come after both of you at any time and in the future for at least 6 more years.
You need professional help to separate your finances and fix any tax problems that you can fix without his information. If you end up being responsible for penalties due to his not filing, you may need to include that in the divorce proceedings as part of a settlement.
