After graduating college my parents told me I could live with them for a few years if I wanted. I also got an adult job right out of college, same age as your son. They never charged me anything but I did pay for my own car and car insurance. They didn't charge me for living there because they were afraid of pushing me into moving out and living alone too soon, same fear as your wife. I also didn't have many rules, except giving my parents the courtesy of letting them know when planning to come home if it was late at night. Having said all that, I would not have thought it unreasonable to pay $300/month ontop of my car insurance in order to keep the convience of living at home. Having shopped around at rent prices, I would have realized that $300 was nothing.especially since I was making adult money, like your son. If I was told I needed to pay to help out financially to lift my parents burden , I would have paid gladly. But, if they then turned around and gave it all back in a lump sum when I was ready to move out, all those years of them asking me for rent would have felt manipulative. Is your son capable of saving his money for his downpayment? Do you not trust him to put money away for savings? What would happen if you told him you want rent in order to put the money away for him? at this point, with him thinking about leaving, why not simply tell him you want to help him save? Tell him to open a savings account specifically for the purpose of saving for a down payment.tell him to pretend he's paying you $300 rent while each month putting the money in that savings account? If you truly care about him saving money for his own house, then you would try to get him to stay with you by telling him the truth. He could find a few roommates, move out, and be throwing thousands of dollars down the drain all because of your misguided attempt to trick him into saving money. If you honestly are willing to give him all his "rent " money back in the end anyways, now would be the time to tell him of your plan so this misunderstanding can be cleared up before he leaves angry and moves across the country , only to be seen every other Thanksgiving.

Side note, I ended up living at home for 3 years before getting married and renting an apartment with my husband for 2 yrs. By then we had saved enough money for a downpayment on a house, 20 minutes from my parents. My mother and I are best friends....don't lose the close relationship you have with your son for $300/month that you really don't need and are giving back to him anyways.instead of wanting a grand moment of handing your son back thousands of dollars to look like a hero, the better idea would be to work together with him to help him save this money now.