A few people have mentioned that we should definitely delay and others say to send him in and schools cope well with all abilities. I'm so torn!
I completely agree with almost all reasons for delaying children. I think the English education system is poorly designed and detrimental to our children. Also, England is an outlier to most other Western countries. I am Scottish and started school at 5.5 like many other children. Delaying in Scotland is normal and expected. Having children going through the English system is challenging as there is so much judgement on delaying, unlike almost every other country! Because of that I think a real downside to delaying is giving your child a 'difference'. I don't want him to be picked on or if he does well I don't want him or others to say it's only because he's "held back a year" (not my views, just what I've seen others say). I also don't know anyone who has delayed so it would be a big decision.
In addition, my son does not show any interest in academic learning like reading, writing and numbers. A few people have said their summer born was learning to read or desperate to go to school etc and I can absolutely see that being the case with DS2 who is also summer born. But DS1 has always been 'different', very active, very, very emotional, totally distraught if he has to eat certain foods or something touches his hands or mouth, used to smash his head in frustration so hard he'd cut and bruise. He injures himself constantly and I do mean constantly and is sensitive to pain even when it's a small bump. So those are reasons that lead me to think delaying would help him mature and maybe become more resilient before starting school. Particularly this could benefit him in secondary with exams and friendships.
However, DH and I also feel that he's likely to dislike school at any age but excel in sports. He's been physically advanced from a baby onwards and it seems like all his development goes there! He can easily hold his own in terms of gross motor skills, running etc. That's made us think that sports will be his thing at school and that we shouldn't do anything to jeopardise that, like have him playing out of cohort. If sport is the one thing he loves then we want him doing that with his friends in his year and he may not be able to at secondary (primary we've been assured is fine).
Also, he's absolutely enormous so the typical August born concerns are a bit different. At age 2 he was wearing size 5-6 year clothes. Luckily at 3.5 he's still in size 5-6 so I'm hoping is slowing down a little. DH is 6'6 so he gets it from there. He will ridiculously stand out if going down a cohort and that could lead quickly to children in secondary asking if he's out of year. Again, we're worried about bullying and him feeling different.
Lastly, we don't expect DS2 to need to delay as he's a completely different child. He's so different that he almost makes me worry about DS1 even more because if DS2 is the 'normal' child then how will DS1 manage with all these other children who can sit and focus? DS2 has much better attention skills already at 20 months than DS1 has at 3.5. So how do we justify delaying one and not the other without DS1 thinking his parents thought he couldn't cope or that his brother is better than him?