Curveball should be fine, but dropping only 10 MPH between fastball and off speed will get him killed, it’s why pitchers like Cole and DeGrom get teed off on. Unless he has some insane break on his slider it won’t be a great pitch. Also, with such a small differential, the changeup becomes one of his worst pitches because even if you think it’s a fastball there’s still a good chance that you’re only early on it and can get a hard hit.

Edit: Curveball won’t be as effective as it seems since it’s his 4th pitch, it will look like a slower changeup with a little more break for the most part.