Hi Piglet
re your comments in quote marks:-
"FIL is still around and even though him and MIL have been married for over 50 years and live together they aren't together in the true sense of the word, they lead separate lives. FIL is great and very supportive but is a quiet man and doesn't tend to get involved in arguments. My DH is much closer to his Mum than his Dad and often him and I disagree about him favouring his Mum over his Dad. I get on well with FIL and feel quite sorry about the way she treats him at times"
I would not feel too sorry for your FIL; he has also played a role here in this overall dysfunctional family that your DH is still a part of. FIL is a weak man and a bystander and they act out of self preservation and want of a quiet life. Its of no surprise to me that they live separate lives; if MIL is indeed a narcissist such women cannot maintain any relationship at all. Men in such relationships are often kicked out or are narcissistic themselves. FIL and MIL get what they want out of this relationship; that is why they are still together.
"DH has become a lot better at standing up to his Mum, but he still panders to her for an easy life. He doesn't have contact with his siblings, mainly so he doesn't upset his Mum. I know for a fact that one of his siblings is definitely not in contact with their Mum because of her behaviour towards his partner. The other one has tried to be in contact as he wanted his children to have their grandparents in their life but MIL refused, probably because he doesn't do what she wants"
He still has a very long way to go because he still panders to her and seeks her approval (approval she will not give him, its all conditional). He still has a very dysfunctional relationship with his mother. The second sentence here is appallingly bad. His siblings have backed off from their parents for good reason. It is hard being the last one left (in that he is the only one who now bothers with her) as your DH is but he needs to realise that his own inertia is hurting his own self as well as his own family unit.
"DH is supportive to me and backs me up a lot more than he used to but feels caught in the middle and just wants a quiet life. DH mainly gets upset with his Mums behaviour but on occasion feels that I could do more to keep the peace. I am generally very assertive and feel that I bite my tongue a lot more than I would with some people therefore I'm doing my bit".
Wanting a quiet life is the role that a bystander plays and he is playing that role to perfection. Your DH I am sorry to say is weak (like his bystander dad) and he is also a large part of the overall problem here. He is very torn between his mother and you as his wife and certainly has divided loyalties.
I would keep your child well away from his mother going forward.