It's the first time I have owned a Siamese kitten, and I am not sure whether he should be allowed to go out the same as our other cat who we have had for 10 years now.
I am really worried that he will get lost or taken. He is 10 months old and I only let him out in the back garden. Any advice would be most welcome.
p.s. I have tried a harness but he just drops down as if he has been shot.
This is a difficult question, and I really feel for you as there are so many factors to weigh up.
We in the UK are more used to letting our cats outside than in some other countries (in the United States, many people wouldn't contemplate letting a Siamese cat outside, and most American breeders insist that kittens/cats must be kept indoors at all times. Some UK breeders feel this way, too.)
A lot will depend on the type of area you live in. Are the roads busy? Is the area generally safe for cats? Are there foxes? How has your other cat got on outside?
Many people would say better safe than sorry, and that your kitten won't miss what he's never had if he's never let outside. However, cats do need some fresh air and sunshine (which provides them with Vitamin D).
Certainly, while your cat is still a youngster, I would only let him out under supervision, and then only if you can be sure that he can't escape. Even if you get to a point where you're letting him out without supervision, I would always make sure (this goes for any cat) that he's only let outside during daylight hours, and kept indoors once it gets dark.
Siamese cats are intelligent and smart, and left alone will generally find their way back home. However, they're also curious and adventurous, and prone to investigating sheds and garages, where they may get shut in.
A harness would be a good way of allowing him outdoor access with you. However, most cats won't take to a harness readily, and training takes patience, persistence, and treats! The important thing is to go at your cat's pace, not yours, and practice indoors until you're sure that he won't panic and back out of the harness (which happened to me once.)
Alternatively, would you be able to build some sort of run or enclosure? A fenced-off area with a 'roof' made of strong chicken wire could provide a safe environment where your cat can take advantage of sunny days while still being safe.
(Depending on your budget, there are companies which build specially-made, escape-proof enclosures for cats.)
Ultimately it will depend on what you feel comfortable with. But if you do decide to let your new cat out, microchipping him is, in my opinion, absolutely essential to increase the chances of him being found and returned if he does happen to get lost.