Getting married to someone who owes the IRS?

Prior to our relationship, my fiance did not file taxes for 10+ years; he and his ex were independent contractors and didn't put aside enough money to pay their taxes at the end of each year. They have since filed all of the missing returns but could not make payment, which was estimated at $300,000. For the last 5 years, he has filed (and paid) his taxes on time.

3 years ago, the IRS contacted him regarding the $300K. He provided them with documentation regarding income and referred them to his accountant. We have not heard from this since then.

I own my home out-right and have unearned income (and 3 children) to protect, but I cannot afford to pay his tax bill. His ex-wife has significant income, but she will not communicate with him about this debt. (She filed an Innocent Spouse claim 3 years ago, which was immediately denied.)

We have not gotten married for the last year out of fear that the IRS will come calling and my finances will be unprotected. Is there NO way that we can get married? What if we do not mingle our finances or own anything jointly + a prenup designating my income as solely my own? It is so hard to just wait this one out...