Siamese's urination habit

I have a Siamese cat whose name is Dexter. He is approximately 10
years old. We welcomed him into our family when he was about 3 or 4.
He was in horrible condition -- his nails were not taken care of, his
hair was horrible, etc. It took him about 6 months to adjust to us.
We moved after having him about 4 years. His transition to the new
place was great. No problems at all. We moved again at the end of
September. We had to board him for a week because the owners of our
new house had not left it yet. When we did move in, again we had no
difficulties with the cat.

I am a teacher and I was at home for two weeks around the holidays.
Sure enough, when Dexter was left alone again after I returned to work,
he peed by the door. This happened off and on for a couple of weeks
before I took him to the vet. The doctor put him on antibiotics for a
week (although he did not test his urine because he could not get
Dexter to go). He did take his blood. The peeing continued so we took
him in again and this time the doctor was able to get his urine. The
urinalysis was clear so the vet said it was probably behavioral. He
recommended we put the cat in the laundry room (this is the only place
in our house where we could put him in - the bathrooms are too small
and the kitchen does not have doors) whenever we go out and also at
night. The problem is now that Dexter is retaliating. After being in
the room Thursday night to Friday afternoon (husband worked 6am to
2pm), he ran up the stairs and peed on my son's mat that is under his
chair. This morning we had to grocery shop and when we let him out of
the laundry room when we got back, he'd thrown up several times in the
room.

My problem is I think that putting him in the room (which is supposedly
supposed to retrain him) is making him hostile towards us (hence the
peeing in the dining room) and probably stressing him out (the throwing
up). I plan to call the vet again tomorrow to see if he has any other
ideas. We do plan on taking up the piece of carpet in front of the
door and putting linoleum (we'd planned to do that even before that cat
was peeing there). Hopefully that will get rid of the smell and cut
down on his instinct to pee there.

Is there anything else we can try? Any tests or problems the vet could
have missed? He did say that Dexter had lost 5 ounces in two weeks.
(Now weighing about 9 pounds). But the blood and urine was clear.

Thanks,

Tiggerific