At 5 weeks out, you should be having less problems eating, not more. It sounds like your surgeon's staff is doing too good of a job protecting him, and he really needs to know what's going on and address this. It could be stricture (not common with the DS but they do happen) or an ulcer. Bottom line, it isn't something your pcp can address.
regarding the potassium, if your potassium is "extremely low" you will need potassium supplements to fix it - this is little to no potassium in vitamin pills, and not enough in foods to fix this, even if you could eat a lot of potassium rich foods, which at the moment, you can't. This is something your pcp could prescribe for you.
Call the surgeon's office again. Insist on being seen. Either that, or head in to the ER and get the problem evaluated. I totally understand that no one wants the misery of an ER visit, but there is something wrong and someone needs to figure out what the problem is and fix it.
PS they can also give you potassium in the ER if needed, and can give it IV if you can't take it orally.