My current thinking is a bit on the darker side. I see Guybrush as a man with very severe issues who has lost himself in this endless fantasy of Monkey Island and the rivalry with his nemesis LeChuck. The ending of Monkey Island 2 was ALMOST the point at which the illusion was broken, but he instead created an even deeper fantasy that we see in the prologue of Rtmi. I think his son and his friends do not exist, they are merely extensions of his own personality that revels in the stories of pirates and adventures and act as a conduit for him to become lost yet again in his fantasy.
I mentioned in a different comment that Elaine, in certain sections of the story, acts almost like a therapist, guiding him along to face certain truths about himself. Her very peculiar behaviour when (and if) you choose to leave the amusement park is almost like she is waiting for Guybrush to make the choice to step out of the delusion. I find the scene on the bench at the end striking in that it seems like it’s the final moment of truth. Guybrush is almost free, but there is a final test. Elaine whispers about a hidden treasure and the promise of adventure. Then Guybrush is sat alone (maybe he was always alone). Does he finally break free of the fantasy, or does he cave in and meet Elaine at the dock? I guess that’s for us to decide.