Poor child, it must be awful for him, watching his wider family and seeing the way the other children are loved and respected and cared for, and knowing he doesn't get that same treatment.
It must be horribly damaging for him.
He probably thinks that he's not as special as his cousins / siblings, as he's the only one who is treated this way.
I think you're right to acknowledge that you cannot continue like this. All the help you and your family give is just sustaining this woman's lifestyle and enabling her to horribly neglect a poor little child who doesn't deserve to be treated like this.
Your help isn't helping the child if he's so malnutritioned his growth is stunted, that's incredibly shocking, and shows whatever level of intervention you and the rest of the family provide, well, it's clearly not enough. I know that you could think that's a harsh thing to say, but you have to get past your own feelings and look at this child's life from his perspective. His siblings and cousins are never abandoned and starved. HIs siblings and cousins have family who love and care for them 24/7. And yet... this child does not have even the basic security of knowing any one single adult will be there for him and put him first. This child is being neglected and abused in front of you all. And I wonder if he will ever understand why. Why when it comes to him, no one helps him and no one will take responsibility for him.
He's lying awake crying for his mum at your house, you are watching abuse happen in front of you. And thank God you've had enough of letting his boy get damaged in front of your eyes. I suspect the rest of the family would have been happy for it to continue ... which is not at all ok as they are putting their own feelings above the plight of a child.
It's so sad that this latest episode of neglect has become about his crying upsetting your children. I know you were focused on how much the child's mother had imposed on you and your family and how rubbish that is, and I agree it's rubbish. And I'm glad you were angry enough about her doing this and the effects on your family to report her to social services.
But what kind of message will this child get? That his upset distressed his cousins and therefore ... what? That he must hide his feelings another time or he won't be 'saved' by you and won't have anyone to look after him? That his little cousins upset must come first, even as he's in awful distress crying for his mother who never came? His mother abandoned him last night. And it must have torn his heart in two poor kid.
Anyway, I'm glad you've done the right thing. I hope social services can find an adult who is willing and able to put the child first. Whether that's the mother stepping up (though she didn't for the other two children so not sure why she'd bother for this one), or whether it's someone else.