You are in the uk the same as me. My son is about to start uni. The debts are going to be quite substantial - however it is my understanding that the repayment rates aren’t massive( ie from student finance England)The banks only seem to be offering small overdrafts etc, and a credit card. So even with all that debt someone ought to be able to afford a contribution such as one hundred a month (or more) if they have been living back at home for three years. 

If that were me I would ask my partner how long the arrangement is likely to continue for. 

I also live in the UK, and have experience of the Uni finance system.  If your SS has a debt regarding a student loan from a UK university, then he is not required to start repaying it until he is earning above a set amount, and the debt repayments are usually set at quite a low figure, repayed over many years.  (My daughters in their mid/late 30s are still repaying theirs but the amounts are tiny).  If SS has acquired debts from other sources then he should be responsible for these, it is not up to you and your partner to be supporting a man who is nearly 30. 

It would do him a lot of good if he was forced to stand on his own two feet and make his way in the world.  I strongly suggest that you talk to your wife with a view to setting a time scale by which he must leave - I suggest 2/3 months which would give him time to make arrangements.   Encouraging him to be a  massive cuckoo in the nest, beak forever open, is not doing anyone any favours. 

This is a tough spot to be in. So many of us here suffer and struggle trying to please our spouses so they can have things in their lives that bring them joy. Unfortunately, you are suffering and even indicated it is starting to affect your mental health. This is not good! Many health issues can be brought on due to stress, as I'm sure you know.

You must approach it with your wife, especially since you've already allowed him to live expense free withyou both for 3 YEARS!! WOW! You're a saint in my opinon.

Why does it have to be that way? Her putting him first? Love for your child isn't coddling and enabling him or her to sponge off of you both, love is setting parameters and boundaries and letting them fly on their own.

Many have given their spouses ultimatums, and timelines to make the necessary changes. It sounds like this is where you have arrived.

You matter! You need peace in  your life. With love, ALL THINGS ARE POSSIBLE. Approach her from a loving perspective, let her know your relationship is suffering as a result of her adult son.

 '... obviously her son comes first and that’s how it should be.'

Er...no. Just no. She is married to you, not her son and he is a grown man who works. So he has a debt problem but he isn't the first or last person to find himself in that situation. You've been more than supportive by allowing him to live there rent-free for 3 years but ultimately, that isn't helping him to manage his money and stand on his own two feet - it's helping him to avoid it.

Student loans don't have to be paid back until you earn in excess of £25,500pa and even then it's only 9% of how much you earn above this figure. That leaves quite a lot of money to be self-supporting as well as pay off debts. Citizens Advice, National Debt line & StepChange Debt Charity all offer non-profit debt counselling help (in other words, a one-to-one session with someone paid to help you, not to make money out of you). A chat with any of these would be a good step in getting him to take responsibility for himself.

You're unhappy and you're being taken advantage of, so this needs to change. Nobody has any right to make you feel like this in your own home. You need to sit down with your partner and make this very clear; the problem isn't going to disappear unless steps are actively taken to resolve it. If she respects you, she should be prepare to accept that you've been more than reasonable but that you're unhappy and therefore something needs to change.

SS won't be paying anyone any time, based on your description of his entitlement mentality.  It is a shame that money your great-grandmother worked hard for went to such an ungrateful source but you can't change that now.  

Keep in mind that this guy is an ADULT.  His day-to-day existence and support should no longer be his mommy's top priority.  When your wife took her vows to you, she should have understood that or she shouldn't have made them.  

Next, you must have a conversation with your DW about this.  As hard as confrontation can be, it is better than the simmering resentment and unhappiness brewing in you. That's not good for you, not good for her, not good for your relationship.  Start out by first asking her to describe and clarify her position on supporting her son - "How long do you think SS should continue to live with us?  Do you think supporting him long-term is healthy?  What is the best way for him to become self-supporting?  Do you think he should pay back money he has borrowed?" 

This will give you some insights into her thought process.  If she cannot explain or answer these questions, or becomes immediately defensive, you've got an answer of sorts.  She expects it to go on indefinitely and she expects to coddle him for the rest of his life.  

This is not acceptible, though, and you will have to draw a very deep line in the sand.  This is YOUR home, too, and you have a right to what happens in it.  You don't need to have it in the same conversation but maybe a bit later you can say you've thought about your DW's answers to her son's situation, and you don't think it is something you can tolerate any longer.  You can then ask her what she thinks you should plan to do, as you will no longer be willing to live in the same household as an adult man who should have been on his own a long time ago.    

If she tells you that you should plan to move out (because she won't ask her son to)  then you also have another answer.  You are not her priority, her ADULT son is.  

The most fatal of fatal flaws in any blended family marriage is the kid "will always come first to my partner".  Any time one partner puts anything or anyone above the marriage and the other partner that is the end.  The marriage may not know it is the end, but the marriage is over. It is just a matter of time.

You are young.  You are of a comparable age to your wife's son.  Compare yourself to this toxic POS non adult and note how massive a failure your wife is as a parent.  If her child comes first, and she has failed that miserably at what she clearly considers her priority what is appealing enough about her to stay in this travesty of a marriage where you are not your partner's priority, your marriage is not your partner's priority and you are subjugated to a rude, manipulative, incubus of a marriage sucking and soul sucking asshole adult  step child?