Has this child actually told an independent person that he does not want to see his mum? He could have been set against her by propaganda from Dad or he might be genuinely scared of seeing his mum.
It is all to easy for a controlling parent to say all this about a child and often the poor child dare not contradict his parent.
As she has supervised contact has this been set up by a court order, in which case it could presumably be challenged.
A difficult situation indeed. Such allegations of "he doesn't want to see you." by a parent are commonly made and trying to find the truth of what is happening is a great deal harder.
Being quizzed by chemist's assistant in Boots.