It would help to know what his real ceiling is.
He probably will be inconsistent, with a higher-than-average OAV, but with a decent number of Ks. So maybe he has a 1.40 WHIP, and if he's lucky (avoids the 3-run HRs) then he'll give you maybe a low 4s ERA. If he's unlucky, then maybe he gives you a mid-5s ERA.
Also with a guy like this, you should pay attention to how he does vs. ies and vs. righties. I've seen guys like this who are quirky, and for no apparent reason, have a wide gap between the #s vs. ies and righties.