You need to keep your eyes peeled and note down anything for future reference. A lot seems word of mouth aswell.
If he starts any activities then they may offer holiday care eg DD's ballet school runs a summer school 9-3 for £75 a week. Football clubs seem to offer the same deal.
Her school runs a 4 week summer school £20 a day.
Staggaring leave between you and DH. We are going away Oct not summer as school offers childcare in the summer hols only so a waste to go away then when childcare is available.
Grandparents. Even if they are not up to full time care they could may be drop and pick up at a summer scheme - our council runs activities for 2/3 hours a day, no use for working mums but that plus granny could work. Details in local leisure centres.
Childminder. My frind sometimes pays a mum (reg childminder) at school £20
a day - her son just thinks he is playing with his friend.
You will make contacts when he starts school so may be able to share with another parent eg if you have their child one day they have yours so you don't need to use a full weeks leave.
Speak to employer abour compressed hours - i'm hoping to do my 25 hours in 4 days over the hols. Under 5 you have the right to take unpaid leave some companies extend this to all employees.
As they get older activity residential holidays/guide camp etc.