I recently put a zucchini chunk in my tank, and I took it out half a day later and it had barely been nipped. And I don't think it was even by the pleco.
That's really cool that your pleco is so friendly...Mine is still very antisocial. Also, quite interesting that your only grew that many inches in that many years...Mine is now about 9 inches from tip to tip and I've only had him about a year, maybe less, and I got him when he was tiny. Been feeding him veggie/algae wafers, and I guess he eats whatever's left over in the tank.
I would say don't bother changing your pleco's diet if he's been fine for 6 years and is so friendly and social toward you...sounds like he's quite content with what he's got.