Okay, thanks for all yur advise. He was running around again last night, and doesn't seam to be too bad.
I don't think its seperation, Im just looking after him for my brother (he only has this 1)
He had a fairly sound night too, So Im hoping he is okay. I'll post ack if the situation deteriates!, many thanks for all your help.
If it is anxiety, you my want to try getting some article of clothing, or a blanket or towel, from your brothers apartment. Usually its best to take it over when y move the animal but it may help. Scent is a powerful sense and it could help kitty adjust.
Is this cat normal an indoor/outdoor? If not, letting him out is a bad idea. That much new input could actually kitty sick.
I've used a tiny dollop of pepto bismol on cats wit the runs/stomach issues. There's some controversy Right now as to whether this is a good idea, but if done once or twice over day or two, it shouldn't be a problem.
Give him his space to wander and pay attention t him to let him know he isn't forgotten. Try not to allow the household to get to loud or noisy, as cats don't react well to noise under the best conditions.
Otherwise, you may just have to take him in for a vet check up if kitty doesn't improve.
You didn't mention how long this cat has been with you. It could be seperation anxiety.