You should not mention your stay as it may confuse things. For now, treat the 2 stays as unrelated.

The process for booking a stay for someone else using points cannot be completed via the website, you need to do so via customer support. More details can be found in Booking With Marriott Bonvoy® Points for a Friend?. You cannot reliably book a stay for yourself on points and then "transfer"[1] it to someone else by adding their name to the reservation, some hotels may accept it on arrival but others may not.

There are widespread problems with account theft so a booking made with points requires the account holder to be present when checking in, unless it is a booking made by customer support following the process described in Booking With Marriott Bonvoy® Points for a Friend?. As it stands, with the points booking on your account, a hotel following the Marriott rules strictly will turn your son away.

[1] The language Marriott use in the help article is a little confusing. You can add someone else's name to a booking for various reasons and if you contact Marriott support and ask them to add someone else's name to a booking, they'll do so, but that is not going to transfer the booking to them. You need to specifically mention that you are gifting a reservation to someone else using your points. Marriott customer support has a specific process (that involves verifying account ownership) which I think includes filing a case internally (but at a minimum adds the M8 code). So you need to avoid using language like "add my son to my reservation" and instead specifically state "I am booking a reservation for someone else using my points". The outcome will be that Marriott add your son to the reservation (which is why the help article says "[...] add the recipient's name [...]") but it will follow a different process that will result in your son being able to check in (because the booking will have the M8 booking code).