Since Mom is not responding to your direct concerns about her childs lack of willingness to feed himself, I would just tell her that he needs to be self feeding and give her a time line you feel is appropriate. She is obviously not interested in taking the reigns so to speak and promoting self feeding techniques to her child so you will have to.
If this child refuses to eat at first because he is not being fed, so be it. I wouldn't even say too much except that the food is yummy and he can start eating like the rest of his friends. Just ensure he is well hydrated and when he realizes he is expected to fed himself because you won't he soon will. Be prepared for a few difficult days till he gives in and starts to feed himself.