Not a very polite response.
If you have a copy of the burial record and/or death certificate showing his date of death and age (85 according to the deaths index at SP), you could send it to them and suggest they brush up on their arithmetic skills.
Better still, if you can find the matching birth record, send that too.
Failing that, complain direct to FindAGrave itself (though TBH I wouldn't hold out much hope of them paying any attention).
Or add a new record with the correct information.