OP here. There have been some useful replies to my post, particularly the recommendations of books and on-line resources. Thanks to everyone who posted these. I have already followed up on several and have arranged to spend time with the kids regularly doing science experiments. I plan to focus on explosions, smoke and bad smells which, I think, are the things to keep kids interested. Once they've seen an explosion etc, I hope they will also listen to an explanation of what caused it.
I'll respond to some of the things that have been said here to clarify what is going on:
1) The boy's parents are hippies, not religious. He seems unusually bright to me, but I am obviously biased.
2) There are a lot of home schooled kids in the area and several well-attended support groups. These groups are mostly social, not educational. This area has perhaps the highest population of "alternative" types in the UK.
3) None my grandchildren are forced to learn anything they don't want to. Forcing children to study stuff they can't stand is, IMO, abusive and a waste of everyone's time. My eldest grandchild is 12. She took to reading suddenly a couple of years ago and now reads at a very good adult level. When kids (or adults for that matter) want to learn something, they do it quickly. The eldest grandchild has decided for herself to attend high school to prepare for national exams.
4) At school, I did well in all sciences except chemistry. I don't believe I lacked aptitude, I believe if you wanted to put kids off the subject, you couldn't have done a much better job than the sadistic, boring teacher we had. That was about 45 years ago. Have things have improved? Or just changed?