Investment account jointly owned but reported under one spouse's SSN.

It would seem to me that since the account has his social security number he is responsible for paying taxes on the income related to such account. The brokerage house issued the Form 1099 to him. I do not believe that he can issue a Form 1099 to you. He is not a bank, brokerage house, etc.
From a tax standpoint, you probably have to do nothing, at least at this point.
The real question though is if this account is truly joint and it is presumed you are no longer together or in good standing with each other, why not divide the account equally between you. If the answer is that it is truly his assets then you would not be liable for any related tax liability.

Hope this helps


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