Has my son got any rightsMy sons ex-wife has custody over

Has my son got any right's
My son's ex-wife has custody over their 9yr old daughter.Two month's ago he went to pick her up and her partner came to the door telling to eff off he was no longer welcome and never to go there again'my son was not told why.He came to me and asked if I knew why I told him it was because of him calling her name's ***** ***** had upset her,but they should have told him as it was not my place.My son said he was only joking and didn't realise he had upset her.Since then his ex has told the school who have told social services and the only time he can see her is when she come's to me and he has not to be left alone with her.
I cannot understand why this is they say it's verbal something (not abuse) My son has still not been informed why she can no longer have sleep over's or stay with him without me being there I get message's from his ex to tell me to make sure I stay with in ear shot
Please can you advise thank you Marilyn