Zorro, if this is worrying you too much for you to keep out of all money discussions between you husband and his children, then as I see it, you will need to try and talk to your husband about the whole thing. Don't be lead into discussions with any of your step-children.
As you describe the situation, I cannot help feeling and I apologise if this offends you, that your husband is not being fair to his daughters. Whether or not the money his son has received is a gift, an investment or a legally drawn up loan, I can quite see why his girls are hurt, and perhaps worried that their brother is not going to pay the money back. Unless the loan or whatever it is has been taken into account in your husband's will, it strikes me his son is getting money at the expense of his sisters.
My aunt made to me the mistake of lending my sister's three children money as legal loans when each of them wanted to buy a flat. My eldest niece meticulously paid back her loan at the stated times and in the stated amounts. Her sister paid as and when it suited her with neither shame nor apology whenever she was late paying an instalment, and never repaid the entire sum she had borrowed. My nephew the youngest of the three never paid back a penny. Both my younger niece and my nephew were in a position to have repaid their loans, I may add.
When my aunt died it transpired that she had left legacies to my sister and myself and the bulk of her estate to my sister's children jointly with the proviso that they were to sell her flat and divide the price of it equally between the three of them. No mention was made at all of the money she had lent them, nor of the fact that only the eldest had repaid her loan.
Not unnaturally my eldest niece felt unfairly treated, as in fact her brother and sister shared their great-aunt's estate on equal terms with her, even although they had "been given" serious money while the good lady was alive because they had not kept to the terms of their loan from her, which their sister had done.
My aunt's money was hers to do as she pleased with, just as your husband's money is his, but unless he intends to favour his son above his daughters he is creating a potentially explosive situation on the day his will comes into force. In the nature of things, he won't be here to be worried by that, but you may well be, so I feel you are justified in trying to discuss this matter with your husband. If you decide to do so, I am sure you know how best to approach your husband about it.