Has the children aids society been involved? Did they express any safety concern? If no, then she does not have sufficient cause to deny your ex access.
It's not unheard of mother's to "alienate" children from their fathers. It usually backfires on them with the right lawyer and custody evaluator.
The children fighting thing is just an excuse for her to progress her alienation agenda.
I think she hates and is jealous of you and some anger issues underlining her actions. And quite possibly just looking to cause problems for you two, and her very own child. I take it she has self esteem issues. Think about it.. why wouldn't she just speak to you or your ex about this in an amicable matter as opposed to "just" denying him access. More of a controlpower thing than anything.
Don't tolerate her non sense and use the courts to fix her up.
If the child went back to Mom's with a black eye, then different story. But even then I don't think children's aid would just "cut off" access.
My lawyer would have restored his access within one week. Ask his lawyer to file a motion and send a letter. If they don't agree to reinstate his access or wish to argue the matter, then your lawyer will confirm the motion date and move forward. Your ex needs to first ask the mother though. Ask very nicely and don't give any details. Just confirming that I will be picking up SON_NAME for my parenting time on X date at Y time. If she fires back with a nasty response, just save it and send to your lawyer. Keep asking every time it is your access. Keep it very short and don't get into any details. And don't respond back to her when she says no. You could use these denied requests to your advantage. If your ex makes no requests, then he has "voluntarily" gave up his access. Don't go overboard harrassing her and be polite.
Once you've reinstated the access, you could use this unilateral interruption of his access to seek further access and strengthen your claims of alienation. You would still need OCL to get the child to repeat the nasty comments the mother is making to the child - this is what helped me finally persuade the judge for 50.50.