Im mid 30's and we married with no money and rented togerher for 2-3 years initially.
Good things are she has agreed to marry him for love, where as if your son was rich either through his own business or a trust fund - youd likely be worrying is she marrying him for his money.
Renting, yes a waste of money in some ways but its a fantastic way to test run being a proper relationship. But if theyd decided to get a mortgage for first moving in together and then realised they dont actually get along doing daily mundane life, well thats hard to detangle and would cause you a lot of worry Renting protects them both from serious commitment. They can have a go at playing house and if it all goes wrong and they realise they dont actually like one another, he can pop back to yours, lick his wounds and reassess things.
I'd try and count yourself lucky. An engagement and renting are very easy to walk away from.
For what its worth ive known 3 couples who skipped the renting stage and went straight to a mortgage but then they realised they couldnt get along in the day to day of life.
In each case took 6 months to a year for the relatipnships to crumble. In one instance there was a couple who had dated 6 years and in that time they did a lot of holidays and dates while living with their parents saving up for mortgage - you know the fun stuff of being in a relationship. Once they actually lived together and dealt with boring parts of life and dirty washing etc they broke up after only 6 mpnths. All those years wasted as they couldnt actually bear to live with on another.
So i think your son and by connection you - are in a good position.
I agree, dont interfere, try and focus on positives and let his life unfold. Just offer support.
Best wishes x