Have only skimmed the thread, so apologies if I'm repeating anything.
We had a similar situation with our youngest son when he was in Yr1.
TBH, although you can't expect every child to play with every single child, this amounts to exclusion, encouraged by a parent, and it's against (our) school anti-bullying policy.
It took an age to get the whole scenario sorted out (was a bigger issue than just this one thing) but all the boys involved play well together now (in year 3).
And much as said mother still dislikes my family intensely she cheered my son on (along with others) at the cubs swimming gala last night, so there is hope for the longer term.