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I was excited to find a small H.plecostomus recently in a shop as they're bizarrely less common than the sail fins around here. Unfortunately despite being in good healthy it's been almost 3 months with zero signs of growth. I have him in a 125gal tank, lots of available natural food and I feed rephashy.
I treated for internal parasites right off the bat and he doesn't have any symptoms, his belly stays slightly round as he's a non stop eater.

Is it just going to be a waiting game? Is he a dud? Did I get my species wrong? I've had fish from the pet store be stunted but dudes only 3" tops.

Yup, just keep the food coming and keep the water quality OK and it'll grow for sure. Treating for interal parasites, depending on how you do it, can clear out their guts and so it can take a while for them to bounce back.

Cheers,

Jools

Sometimes we never actually notice growth and it is happening. There are other factors like diet and availability of food I'd explore. The availability is more of an issue for busy tanks and yours doesn't look too busy.
I agree Hypostomus but other then the H. cochliodon group I'm not a fan of myself explaining the other species that aren't very distinctive. Dwarf species do exist, some not sure we are entirely aware of. But stunting again happens so easily even for the best fishkeepers.

Something I have found over the years is that the key to growth in many lorcariids (that are not specialist feeders) is that volume of food available plays a bigger role than quality of food. If the pleco has constant access to zucchini, carrots, broccoli stems, etc you will see faster growth than if the fish is fed a quality food like Repashy once a day. This also more closely mimics their eating behavior in the wild.
-Shane