My former spouse has been on SSI for quite some time now due to Diabetes. He is nearing the end and I was told that I would be able to collect back child support from his Social Security after he is deceased. I'm just wondering if this is true.

No. You can't collect back child supper from them. And if he was truly on SSI there is no survivior benefit either. The only thing you could do and are entitled to do if there is already a support case with arrears is to collect against his estate.