Have been in similar-ish position. I suspect it’s fairly common. This is what I’ve learned.
He’s an adult. You shouldn’t expect him to tell you everything. Or judge what’s best for him or whether he’s doing what you think he should do. As an adult, he has to make his own decisions and take responsibility for his own life and finances.
If he’s not been honest with you, do you think it’s because he worried you’d ’go nuclear’? He obviously shouldn’t lie, as it destroys trust, but he doesn’t have to tell you anything either. Adults are entitled to privacy from their parents. They’re more likely to be honest if they feel they won’t be judged or ‘told off’ like a child.
Did you agree his gf could stay? Or did he simply tell you she was? If the latter, he needs to know he should ask you if he can invite a guest to stay in your home for 3 months! If the former, then did you set expectations beforehand of what you expected of her as a longer-term guest.
To avoid viewing her as an extension of your DS, and risk treating her like a child, maybe think what arrangement you’d make if a friend of yours stayed for 3 months. What you’d expect them to do as a guest in terms of housework etc.
If you’re finding it a strain having her there, you should have a quiet word with your DS about this and try to find a compromise. Or ask if she could leave sooner. 3 months is a long stay. It’s your home. It must also be awkward for her being there when he’s at work all day. It’s pretty daunting to cook at a bf’s parent’s home.
If I’m honest, it sounds like you’re judging their relationship: him cooking for her, having no social life etc. That’s none of your business. As long as he/she does what you’ve asked as guests in your home, you should leave them alone.
If you feel he should contribute to bills, then work out how much you’d like and discuss with him. If you can manage without his contribution, IMHO maybe give him some time to regroup. Or (without telling him) pay his contributions into a savings account for him to use later.
If you feel he’s squandered your prior financial support, you could consider asking him to pay you back bit-by-bit. But if you can afford it, maybe write it off.