I'd be grateful for any advice 
DS will be 6 in August, one of the youngest in his class 1. He's our only child.
He's been in absolute tears tonight and has told me that most of the time, at playtime, he plays on his own. He's mentioned it before and we talked about asking to join in, starting a game etc and he seemed fine (about 2/3 months ago). But it appears it's no better.
There is a particular boy who he played with a lot to begin with. He's quite popular this boy as well as being a good foot taller and almost a year older. My son adores this boy. But he's really quite horrible to him (sorry to use this term of a 6year old) constantly one minute being his friend, the next belittling him and encouraging others to do the same.
DH witnessed an incident last week where at drop off, this boy and others were teasing him mercilessly about his hat DH spoke to the boys Grandmother at the time but (to be fair to her) she didn't really say much and possibly was a it flummoxed and not sure what to say.
DS is devastated when this boy "isn't friends" with him and begs and pleads with him. I've tried to explain that he needs to accept that it's not nice/friendly behaviour and to make other friends but he says that they all tell him he can't join in. He was sobbing uncontrollably telling me that every playtime he's playing on his own.
Honestly, my heart is breaking for him . I've told him that we will speak to the teacher together on Friday (it's the only day I can pick him up) and THAT led to a whole other sobbing about how he misses me and wishes I could take him to school (I've recently returned to work full time) which makes me feel utterly shit.
Has anyone any advice? I can't bear the thought of it. I actually get on really well with the other boys Mum but I'm a bit hesitant to speak to her as I had to speak to her about a separate incident between her and DS where he had hit him. She was great about it but I'm not sure I can approach her yet again about her son's behaviour.
TIA