I'm with you OP.
I have always been on the more lax end of leaving my kids for an hour or two, but I think over the summer holidays, to leave a 12 year old for 5 full days every week is not a great idea. Apart from boredom, long periods of being unattended and alone at this age can lead to depression and also the potential to get into loads of trouble!
Instead of looking at kids club, I wonder if there are some other things available, eg there is a football 'camp' at the local town football club, 9-5 for a week here. There are various courses and activities on locally, which you could sign him up for, and he can probably volunteer at the local library one day a week if they will let him, my dd aged 11 does, but it is quite informal.
If he had an activity or task each day, then you are structuring it. He could even do some shopping for you!
At 12, learning to get round on the bus is a great thing to do.
To me, a lot depends on your work, how far away you are and how much you are able to be in contact with him, eg he goes swimming (and at most local leisure centres round here as long as they can swim they can come unaccompanied at age 8) texts you to say he has arrived and texts you to say when he is leaving for home. He can meet a friend (with or without parent) at the swimming pool, and they can swim and hang out for a bit in the cafe.