I was going to ask about your partner.
DP and I share out holiday cover equally. We take the majority of our holiday time in relays, reserving one or two weeks for joint family holiday. It is equally his responsibility to drop DS at any activities or friends.
We do have a good reciprocal arrangement with DS's school friends - important to cultivate friendships - I woh f/t, so do mmy share fo sahms in babysitting or having all thier children to play at weekends and taking my fair share of after school play dates when i can. But DP also takes his fair share in this.
They all seem to prefer to go to holiday camps / activities if a friend from school is also going, so ask around and see if any of his school friends would like to go to any camps on offer- they may be resistant to going at first, but a bit of pressure and then they should find they settle in and it's good for them to make new friends.
If you need to drop work and income to cover hols, then again your partner needs to take due consideration of this and be supportive.
But there is no magic answer to this - not without compromise. And they do need company, and even children with siblings need / prefer some time with peers and freinds rather than siblings (see the number of threads at the end of the holidays by parents driven to distraction by squabbling siblings, desparate for schoo to start again), so bite the bullet and propose to some of your parent-friends that you will take thier children for a day if they can have yours for another. Or suggest you can drop off or pick up from hold projects if thier child would also like to go.