We moved house/areas for Primary 1, so, unlike his classmates, DS didn't know any children as he didn't come from the local nursery.
He was also very emotionally imature and quite naive
Adding to that, DD (who has ASD and is non-verbal) is in the same class, which I thought could be a bit challenging for DS.
My strategy was to schedule as many playdates as possible and make absolute sure the visiting children had THE best time.
Not ashamed to say I invested money, patience, my time at it.
The playdates became the stuff of legend (first graders have low expectations
and, by extension, DS became interesting and cool
This was a short term solution for us, but did the trick. He's now in P4 and is settled in his social group.