You have very little time to work on this. Ideally you should have started last year but it's still possible to get everything done that needs to be done.
It's possible to get a degree in fine arts from a liberal arts college or a university, even state universities, not necessarily an art school. Even some of those colleges/ universities regarded as extremely selective offer degrees in fine arts, so it's worth your while to find colleges offering to meet full demonstrated financial need for international students just in case they also offer art degrees.
They would be ideal places for your DC to apply to, but they would require a broad curriculum for the first two years as well as art courses. How is your DC at calculus? Essay writing? MFL? Hard sciences?
You need to choose the schools DC is interested in and read the entire admissions and financial aid sections of their websites. Then contact both admissions and finaid directly by email or even phone. Email is better these days thanks to covid. It also works better with the time difference.
Your DC will need to create a portfolio and submit it as part of the admissions process. He or she will need to produce grades from secondary education to date, including GCSE results, and will need references from teachers. It helps greatly to have a list of extra curricular activities handy, including experiences in art, plus any awards won, etc. The admissions process requires personal essays and all the rest in order to get a full picture of the student.
There may be SAT or ACT requirements (not the British SATs) .
You will need to create a College Board account through which your student will do applications and apply for financial aid.
There are US college events in the UK where the application process and all the rest of it is explained. I would advise you to go to one ASAP.
Talk ASAP (i.e. next week) to the sixth form college admissions person because there is a lot to do. Don't wait until your DC is in sixth form.