The suit was really the first strike. His work history was highly professional, and he didn’t seem comfortable in the group (band shirts, some shorts, very casual). He seemed to have the talent, but there’s no way he’d have gotten along in a world of “status meeting” Fridays (gather in the big conference room around 3:30 to call it a week with some beers).
Dunno, I think I’d still ask if he’s comfortable dressing down, that your dress code doesn’t say “may” but “should” to the informal attire.