I am just sad that no matter what I do, there will not be a guitar surprise this Christmas.
No, no, this is wrong. There will be a guitar surprise for Christmas. It will be a gorgeous, incredible instrument that you DS is going to realize is way, way above what Grandma gave him. He will be thrilled beyond thrilled. And it will, I am sure, be even more of a surprise than originally because he will never be expecting another one, especially a fine one.
Go for it, OP. Nothing has been ruined between you, your DH and your DS. I will lay a thousand to one odds he is stunned on Christmas morning.