Did you both go in front of the judge? Did the judge ask if either of you were being coerced, threatened, etc...? I know our judge did when we were dealing with our custody/visitation agreement and my attorney made sure everything was done in court, in front of the judge for that very reason. My ex was in treatment for addiction and was also a war veteran with serious PTSD and an array of other issues. My attorney made a point that we did EVERYTHING in front of the judge for this very reason...so he couldn't turn around at a later date and say he wasn't of sound mind, was coerced or whatever other reason he could have come up with.
I don't think he has a leg to stand on.