The easy answer is, it depends which 20% of hands he's shoving. If it's top 20% we can't call, if it's bottom 20% we obviously can. Since we have a large sample, we could just plug a range consisting of all the hands he's shoved into pokerstove and see if we have enough equity. Or if we remember enough of the hands where he's shoved and been called we can guess at what kind of hands he shoves and stove against that.

Failing that, we have to guess, and the obvious starting point is what he's doing the other 80% of the time. If he's folding 80%, then we can assume the 20% he's shoving is the top 20% (with a bit of mixed-strategy fuzziness) and we should fold. (You can only call a shove with T9s vs a push/fold opponent with <12bb stacks).

If he's raising 20%, shoving 20%, and folding 60%, and showing premium hands when he raises, we might expect his shoving range to be the second 20% of hands which is almost-but-not-quite breakeven. Etc.

TL;DR: 'shoves 20%' isn't enough information, we need to know or guess which 20%.