When my mom died, there were instructions to deduct the $5K she loaned for my first house, $2k for repairs to my second, before closing and $3k for my car I bought when I moved back to the USA. With their house, she left about a million to four of us. One other sister got money for her house, too. So we had less than my other two siblings.
I say, especially if you have several children and you have plenty to live on, you will do it, but it is coming out of your retirement and his i inheritance. If you need it, he will have to take a second loan on his house down the road. And, when you die, he'll have it deducted and dispersed to his siblings. Just an idea.
Also, make him do the compounding math on $40k from now until he is 60 at a 4% average. He might rather let you keep it for him until you die.
Also, have you had a private conversation with HER parents? Get on the same as them, have a dinner with them snd the kids, and you both tell them what you want to do. Back yard potluck and you pay for a trip around the world or you can put down 50% on a starter house out in Knoxville with it!!! I hope they don't have any debt!!!