I've never seen an aggressive pleco before o.o Aggressive other fish, yes, but never a pleco... though I suppose it can happen since I've seen surprisingly docile betta...
Though if you haven't had him long then you could always try switching him out for a bristlenose? They don't get near as big as regular pleco's so you wouldn't have to worry about selling him back and could keep him for his entire lifespan :) Plus if you get a male then they grow that really cool feature on their nose ^-^
Male Bettas: Jasper (PK), Gold Dust (PK) Iced Steel (EE HM)
Other: Faith, Frankie, Sonic, Ashley & Anna (Juli cory catfish), Mr. Bristles (albino bristlenose pleco), The Shrimples (dozen ghost shrimp)