This story is concerning,
It is a new school year, so it would be good if his teachers and school are aware of the situation so that they can watch out for him.
Head teachers have a pastoral duty to their students, perhaps the head can be informed about these concerns, confidentially,...they of course also have a legal duty of care to the child and are able to monitor his wellbeing.
I think the mother and the little boy are both in need of specialist intervention and care.
I think you are wise and kind to raise this concern.
I hope that you will still have access to the child and his mother, may I suggest treading carefully so that you maintain a relationship with mum, (I hope that does not sound patronising),
It is just that it is better that you are still able to visit, that way you can be of more help to the little boy.
Hoping that mother and child get the right help,
and that you are o.k..