The Texas Death Index must be a mistake then.
I tried the subscriptions I had, but I haven't found an obit for him. :(
Since you know where he was buried (listed on death certificate) then you could contact the cemetery. Perhaps his last wife was buried next to him? The cemetery might also have more info on other family members.
If the funeral home is still in business, then it wouldn't hurt to see if they will share any info, too.
Sorry -- I thought I'd be able to find more info! :(