Just a thought (I don't know if they do) But could you ask if the 17 year old can take some sort of entrance exam at his local college?
I think at age 17 it is probably not worth going into 6th form, I think most people leave 6th form at 18 years of age (they do two years in 6th form from the age of 16, well they did in the late eighties/nineties when I went to school!)