cat64 it's not about legality at all. A child is not legally required to be at school for any days until the term after their 5th birthday. (They aren't even legally required to be at school then, but they are legally required to be in full-time education, which can happen at school or otherwise)
Honestly, ooops, I think the educational value your child will have got from your holiday will outstrip whatever the school was offering in the last week of the summer term.
How come he's done a whole year in reception and he's still not 5? August birthday?