As you say it's essential to prioritise your child's best interests, and as many have mentioned, he may be too young to fully understand the implications of this decision. Consider the benefits of transferring him to a school that provides better support, allowing him to repeat a year and potentially thrive academically. If he stays in his current year group, he may continue to fall behind, with these differences only becoming more pronounced over time. Initially, repeating a year may impact his self-esteem, but as he matures, he'll likely compare himself more to his successful peers, which could be even more detrimental.
Speaking from personal experience, I had to repeat year 12 due to failed exams. Although I initially felt inadequate, I was old enough to realise the importance of a solid education and exam results for my future, and it really worked out. If your child's struggles stem from a disability and inadequate school support, this change could be especially beneficial.
Given that he's a summer-born child, he'll only be slightly older than his new classmates, making the transition smoother. He might be concerned about the social stigma of repeating a grade, but in a new school, this is less likely to be an issue. He'll have a better opportunity to keep up with his peers and gain a strong academic footing.
If he's already fallen behind by two or more years, it's improbable that he'll catch up with his current classmates. Large classes present challenges for teachers to accommodate diverse learning abilities, and as other posters have mentioned, they often don't have the time or resources to develop personalised lesson plans. The current system is more about appearance than substance.
My wife, a primary school teacher, finds this reality disheartening and frustrating. Parents often seek diagnoses for their children, hoping it will lead to meaningful changes, but the lack of resources prevents any substantial improvements. Teachers are left with limited options, such as peer teaching or assigning unrelated tasks to struggling students, which can be unfair and ineffective.
Ultimately, prioritising your child's academic and emotional well-being is crucial, and exploring alternative educational environments may provide the support he needs to succeed.