Scooting

I adopted my 2 year-old male cat, Chester, a month ago. From the first day he was scooting across the floor. No foul smell or poop trailing behind, just scooting. I brought him to the vet a week later for his wellness exam and she expressed his anal glands and was satisfied that was the issue. He was fine for a few days then started again, rarely at first and with continuing frequency towards the 2 week mark after the first vet visit. Around this time he started having diarrhea and loose stool. Brought him to the vet again and it turns out he had tapeworms so I gave him his dose of Drontal and he started eating and acting healthier. 

He's still scooting, though, accompanied by smaller stool that is only about once a day now and is loose but not necessarily concerning (still shaped, but will stick to his bottom). My only thought is something in his food is causing this issue. Right now as per the vets original suggestion I'm feeding him Purina One dry 2x a day which I now realize is not a good quality food. Starting tonight I have higher quality wet food for dinner (I bought Innova, Blue Spa, and Nature's Recipe this afternoon) and am trying to research a better dry food for the morning (I'd like to keep him on dry for when I go on vacation and the sitter only comes once a day).  My question, what ingredient in food would be causing this? What food should I switch to? Or is it not the food at all? I'm at a loss and my vet just told me he's probably marking territory which I don't  buy. 

That's not how they mark, usually.  He may have an allergy to something in the food.  If you've fed him a variety for a bit, try narrowing it down.  For instance, feed only chicken varieties for a couple of days, then turkey, etc.  Check the ingredients labels, too, because there may be something else (additives like guar gum and carageenans) might also affect him.