Have a common pleco in my 90. the tank has a algae problem (always has always will its close to a bay window and only place in the house it can go) had him 3 months and hasn't grown or very very little about 2 inches now.
90% of Plecos in aquariums die to either starvation or maltrution, he will need algae wafers at the least. Boiled Zuchinni or cucumber is even better, but just as a treat.
Don't worry. Ishmael, my leopard sailfin, is up to 8 inches and He's only been in there a year. Your Pleco will certainly grow. The important thing is that you have a great tank size and a million options with wafers and veggies. you probably have wood too, which can't hurt. Ishmael doesn't like to clean the ornaments so a friend suggested a...I believe he said "chinese algea eater." But I can ask again. Your little friend will be a monster fish after just a few years of loving care, algea wafers, and whatever veggies you can get him to eat. Good luck