I expect if you ask around you will find there are some options for at least up to age 13 during holidays.
The holiday club DD goes to, which uses the buildings/facilities of a big private school but is open to everyone, takes 9-13 year olds in the general kids club, and 13-16s for more specific activities (sport, drama, photography, music etc).
The local council also runs quite a lot of sport/multi-activity things during school holidays for all ages up to 16, but they often don't cover a full working day, so unless your DC can make their own way there and back or you only work part-time anyway, they are a bit useless.
There are also things like PGL or holiday activities run by the Youth Hostels Association, if you'd be interested in sending him away for a few days or a week (can get expensive, though - YHA is cheaper than PGL).
Otherwise, maybe taking in turns with other parents to be around to loosely supervise a few of them hanging out at each others' houses? But they do grow up fast, and although your DS might not seem responsible enough to be left all day now, by the end of year 7 he might be.