I would say (hope this doesnt come off as mean, not trying to be, just straight forward kind of person)
1. Get a good liquid test kit if you are going to check on you water. Last longer then strips and more cost effective, as well as more true to reading.
2. A pleco which is a very dirty fish (pooping machine) which at 12in can have an impact on your bioload.
3. Adding that large of a fish it is important to know your water params as it can spike as it adjusts.
4. Feed him some peas (laxative effect on fish) blanch them and squeeze the pea out of the skin.
5. Sounds to me like he has blockage or he is really constipated. I will leave it to the more seasoned pleco owners on to other issues. If it could be a parasite etc.