You can just tell him that’s he’s graduated out of the playpen and is ready for the big boys bed! You can do the whole graduation ceremony, get him a cake, make it a big deal.
You can prepare him for the ceremony about 1 week in advance by telling him everyday that he’s so close to become a graduate and how proud you are of him.
And then on the day of graduation, make it a huge deal!
Also good to buy bed guards as well for the first year :) I have tagged the guards we used
Congrats mama! Cool milestone!