If the dorsal fin ray number is less than 10 and considering the resemblance with common pleco, it might be a Hypostomus species.
Some of these can stay rather small, even smaller than a bristlenose which would explain the lack or maybe slow growth of yours. I think the largest I came across is about 10". A lot of them have a spotted pattern and of course the most famous one is Hypostomus plecostomus
Here is the entire list of pictures of Hypostomus. Have a look if yours resembles any of them.
http/www.planetcatfish.com/catelog/genus.php?genus_id=17
Some of these can stay rather small, even smaller than a bristlenose which would explain the lack or maybe slow growth of yours. I think the largest I came across is about 10". A lot of them have a spotted pattern and of course the most famous one is Hypostomus plecostomus
Here is the entire list of pictures of Hypostomus. Have a look if yours resembles any of them.
http/www.planetcatfish.com/catelog/genus.php?genus_id=17