Husband's adult son is a cuckoo in the nest

Book yourself a week away in a lovely little guest house.
Breakfast served every morning, whilst you peruse the paper...
Leave them to it for a while.

If you do that you may finish the marriage. Go easy.

Tell your husband exactly what you said above - that he treats his son like a child but doesn't want to be the parent, and that you have had enough of being mummy to a child of 26, so you are planning to move out for a while and let him do all the mothering.

Then do it.

At 26, he's a grown man. His room is his own, and you don't need to go in there. Does he go to work? Pay you any rent? What does he do around the house?

Go for it - take back control of your life "don't let the buggers get you down" - an old saying that my Granny used to use - maybe not politically correct nowadays but sums up many a situation including this one!!!

Not trite at all, and you have described exactly how I feel,thank you.

My husband just takes his side everytime, I don't do anything for him at all and t the agreement was that he would look after his own washing etc. However, my husband treats him like a child and doesn't want to be the parent. H e had out stayed his welcome at his mum's despite her deadlines for him to move and I think that it what he will do here too. I have tried to have direct discussions with my husband to no avail, he has a clever way of turning it around to bring me at fault every time.

I am sorely tempted to suggest that you do exactly that - ie look for alternative accommodation for yourself. If you are in the position to be able to rent a furnished flat on a short term tenancy (ie six months) then maybe this is the wake up call they both need - if it doesn't result in your desired response then at least you have made a first step to being independant and without the hindrance of two males who are unwilling to respect your role in their lives.
A trite answer but without you answering many questions about your situation and your feelings about your husband this is the best I can do -

All of the above reasons I have given.

What was the agreement when he first moved in as I presume this wasn’t intended to be a long term arrangement? Why has he not moved out yet? He is 26 and working is there any reason he can’t find his own accommodation? Are you cooking washing for him etc? Possibly you have made the home too comfortable for him? I would consider a frank discussion with your husband stating that you are finding the situation as it is completely intolerable etc and that he needs to address the situation.

Tried that, he just shrugs his shoulders and laughs.

Why did his mum throw him out?

Do you do his washing or does he do his own? Likewise cleaning his room and changing his bed. Can you have a gentle chat with him and offer to help him sort his room a bit and suggest he airs his room each day?