check out
http://www.catsinternational.org/
click on articles and read all the great ones on this website...
we over here in the states are always debating about indoor vs outdoor...most vets here tell you to keep them inside...so it depends on which vet you ask...but if the cat goes out and gets hurt where do you have to take them? the vet right..and you have to pay for their service, do you not?
The Shelters and rescues are crammed full of roaming cats that were out and about, and most get euthanised because there is no room left and the rescues have a limit on homes, so they advocate spay/neuter and keeping them inside, it's taken time but eventually they came around to this way of thinking...check out
http://www.hssv.org/LIBRARY/PDF/Behavior/where cat.pdf
if you can.
I am usually the one standing alone on this issue for keeping cats inside...I've seen to much harm and sadness when they have been let outside. Try the can method, you can also revert to a squirt bottle, the whole point is to make it unpleasant to the cat to want to go out, right now he goes out and finds diff things and it's rewarding, just like a dog that gets loose.. they find stuff also and want to get back out of their yard.