Even if May technically counts as summer born, it's surely not 'summer born enough' for it to make a difference unless he was born very premature.
Reception classes in my experience are very aware of the fact that they are receiving children that are only just four, and they do cope well with it.
My dc are both late August born, and I'd say they have to have been really quite significantly behind in their development if they couldn't have coped with reception. Saying that though, at the time they started school they weren't required to start until the January, so they had that extra term of being the oldest at pre school which I think was good for them. I really don't think they would have suffered if they had started in the September though, and it may even have had its benefits.
They always achieved well at primary so they don't support the statistics about summer borns achieving less.
By starting your ds so much later, you also run the risk of him not being stretched enough in future years if he ends up being well over a year older than the youngest children in his class.
Remember that you don't have to start your child full time straight away when they start school. You are allowed to just do four days a week, or just do mornings, and lots of schools have very long settling in periods anyway.