D.S.
I agree with those who have suggested telling your stepson that you would like to give him $200 to use for his rehearsal dinner, making sure that he knows that this is exactly the same as you gave your daughter for her wedding.
However, I would urge you to keep your explanation limited to ONLY the fact that you are unable to offer any more than that. Do not be defensive. There is nothing to apologize for, because you are still giving him an amount of money that is a sacrifice at this point in time, and you should give it with an attitude of love for him.
If you make an issue about your stepson making more money than his father, then that is likely to become the main theme of any conflict that develops. When men feel like someone is blaming them for doing something wrong, they often react badly (even when they ARE WRONG!). His earnings are irrelevant to how much you can afford to contribute, because if he didn't make much money, you still couldn't afford to pay for more.
You can offer to host a nice dinner at your house or the church fellowhip hall that is either homemade or inexpensive take-out, with a budget of $200. (Doesn't have to be fast food, there are other inexpensive options. Sam's and Costco have good ready made entrees.) He can decide for himself whether he'd like to choose that option or come up with the rest of the money for something more expensive.