I have a similar issue with one of my kids. as much as we want friends for them there are people they are better off not associating too much with.
Be prepared, your son most probably will NOT forget about the call and will ask you to follow up.
Consider calling the mother (when you are very calm) and ask in a friendly way if perhaps there was a miscommunication, maybe her son didn’t give her message across properly, etc. Practice your opening lines so you don’t sound desperate or angry or like you are going to cry. If she is not immediately hostile or unfriendly, invite her son to your house, where you can supervise the boys’ interaction (to see if her son might be mean to yours). Also, she might be nervous to host your son if she has no experience with his disability. If it is feasible, and you think this friendship is worth pursuing, would you consider inviting the other kid’s family over on Shabbos or yomtov to get a feel for each other?
Have you discussed with your son’s teacher from previous years which boys might be good candidates for playdates with him? Usually they can tell from the kids’ interaction in school who would be a good shidduch and who to stay away from.
Hatzlacha.