If his IQ is actually 87, that is considered Low Normal, thus no excuse. Even 80 would place him in very mild intellectual disability. Either way, people in that IQ range can have jobs that aren't too complex, they can usually live on their own (with someone to check in on them once in awhile). None of that would prevent 95% normal functioning.
If that's true, it can be proven 100% with a simple DNA test (which he would never do).
Easily provable with a simple blood test (which he will never do).