It is very common that some children have good experience, but some have bad ones. In overseas, studying high school is very different from studying university. Students interaction is more in high schools and bullying usually happens. So, parents and their children need to take more risks if the children study at high school.
As another parent said earlier, you and your child may set a deadline. If no improvement by the deadline, then you can arrange your child to come home.
If he cannot adapt to the UK, you may consider Australia. There are high schools allowed to recruit overseas students. Overseas students are usually living homestay unless you have relatives or friends who can look after him. These schools usually attract students from Asia, such as Korea, Japan, Taiwan, and Singapore. Year 9 is still okay as students are still young and he can still meet some good friends. If he cannot make good frienships with local students, he can still know some from overseas.