I really would talk to some other clubs about what they would offer your son. Not because of the cost, but to get a clear idea of his potential. 10 is quite late to enter a competitive squad system, and given how intense the training is likely to be, he could finish up with a huge knock to his confidence if he isn't able to catch up reasonably quickly.
He could, of course, do it for fun, but 9 hours a week is more training than any of the non-competitive groups I know - seven is the most for the non-elite boys at DS1's old club, which had multiple streams and various levels of competition.
The problem with coming in late is that there is a national system of development that all serious clubs follow, with the boys taking a grade each year, with the end aim being to reach a level to compete at British championships and similar events. Most boys will have entered at level 1, which is Under 9 (or even at Underage level 1, so Under 8) and, by the age of 10, will be at level 2 (or possibly even level 3, birthday-dependent). So if your son is going to be expected to compete, then he's going to be expected to master some hard skills very quickly. It's possible - I know a boy who started at 9 and made phenomenal progress and is now one of the best in the area in his age-group - but his coaches were very clear that this is highly unusual.
I would be trying to find out what path they see your DS following, and how ruthless they're likely to be if he doesn't progress quickly enough - and what other options there are for him.
It may be that he really is that good, and this club are going to work hard to get him to where he needs to be - but I'd be inclined to try and get a second opinion and some clear predictions about where they see him going before committing to an expensive, and quite possibly soul-destroying process.