Stress? I just don't get it

So I decided to take Parker to my parents for 3 days, since he doesn't do well without me and he loved it! As soon as he smelled his scratch post there, it was like he remembered the house. He didn't hide and even ate right away. He didn't use the litter box for the first 24 hours...but did eventually go and all seemed well. However, he was still licking a lot. Same places he's been licking where he is now developing bald patches but he wasn't having full on fits with the floor licking and back rolling. I brought him back here today. He smelled around, got reacquainted with Buddy and immediately had a fit. I just don't understand what it is. Could it be from the cigarette smoke here? Although he spent the first 5 years around it...maybe stress from our roommate that he doesn't like? I don't know what to do. I don't want to give him to my parents because he is too attached to me and they really can't afford a cat and I don't want to have to move back there with him. I am still going to take him to the vet very soon...but this is getting frustrating. I love him so much and don't even want to THINK of giving him up but I need to find out what to do.
You think if its just stress from being here, that medication might work? I'm so torn.

Give him a couple days to get reacclimated. Often it's not the first move that shakes them, it's coming back to their old home again. All of his scent markers were three days old, which probably confused him. Also, even three days apart is a long time for cats, and he will need to get used to having a nother cat in the house. Someone had this same problem around thanksgiving, their cat was fine at the parents' but back at home the cat started acting out.

No matter how resilient Parker is, that's still a LOT to be moving around a cat within a week, and the total stress probably built up until he started acting strangely.