If it's any help- my 2DSs were May born. Both struggled in early primary. If both could have repeated either R or Y1, it would have been a blessing; acknowledged by us and the school/s. But, no. Not an option.
The soldiered on, (alongside other boys, largely, who would have benefitted from being 'held back').
Both mine ended up 'staying back', ultimately; eldest by by being very '15' at GCSEs; then AS levels, nose-diving in Y12; (restart -BTEC this time). So started uni at 19, thank heavens. Now 23, just graduated with a first class in software engineering.
Other stayed the course but, blessedly, did an Art Foundation year (uni v keen for them to have it); now 21, in EU uni for a semester. Future looking good.
It feels like hell at R and Y1 as it all seems disastrous; but I imagine few parents will actually pursue the deferral; thus your child will, like mine (when deferral wasn't an option)- end up in classes with similar DC, being taught to their abilities by people who know what they're doing.
It will almost always work out.