If it is constipation driftwood may help, sometimes Pleco's will chew on driftwood and get extra fiber from it. I am not sure when you are adding the food but Plecos are nocturnal and unless they are starving only feed at night. I know I can put a few brussels sprouts in mid-afternoon and nothing will be touched when I go to bed, by morning not a trace remains. I read somewhere in the past that lettuce can leach something into the water and therefore should be removed if not consumed in a few hours, but brussel sprouts can be left in the tank overnight. I have never done the shelled pea thing because I think it too much of a pain to shell peas. I feed them algae wafers as a staple.
EDIT: It's a rather large Pleco and therefore can live a few days without food, since you just put him in a new tank Friday maybe he is just stressed due to the new environment and doesn't feel like eating yet. I wouldn't go out and buy every type of food available to try and get him to eat just yet, watch him closely and make sure you're feeding at the right times. Unless you notice any other symptoms or odd behavior I would wait a few days before I got really concerned.