You wanted to treat your sons equally, and neither has done anything to you to change your relationship with them. It seems a shame to me that you may profoundly alter your relationship because of how other people have changed – why let your ex-husband’s new wife’s spending habits change how you treat your sons?
You say that your money might end up being all your biological son inherits given the new spending pattern of his father and new wife. You haven’t said, and quite possibly don’t know, about the new wife’s finances, but it could turn out well for your boy. If the new wife has assets, dies and leaves them to your ex, that swells what he could leave to his sons. This is the reflection of you leaving some of your wealth to your stepson, is it not?
Imagine how uncomfortable it would feel if your ex-husband asked you what you and your current husband were planning to spend during your lifetime. He only wants to know so he can judge the consequent size of your biological son’s legacy so that he can adjust his will to balance out your frugalness/overspending when he divides up his estate. You would have every right to be incensed.
I know that if I were in that situation, I would want to tell him to mind his own business. I might well be motivated to spend the lot so my ex could not consider our son catered for and bequeath his assets to the new wife and her family!
Money is for spending. It is a tool of exchange. We hand over money and buy things, services, time, and we have every right to decide as individuals how we spend our own money. If your husband’s lifestyle has changed because of the company he is keeping, that is entirely his business, as far as I can see.
There is also the point that expecting an inheritance can cause unintended consequences. Some potential beneficiaries lack the inspiration to create their own wealth, spending power, choice and independence. The thing is, when you get someone else’s money it can be accompanied by a sense of how it should be used, even if there are no explicit rules – as there are in some cases, like trust funds. Clients say things like “I am going to use it to invest for the future because dad would turn in his grave if I blew it on a holiday”.