I think you CAN miss the game.
Its fine... do this for your friend....
Your Husband... should understand...
Your girlfriend is moving OUT of State...and so, this is a last full day you can spend with her.
Have your HUSBAND go to the game. HE should be there... for you.
Your son can understand.
My daughter at that age, could understand if I could not make it.
It is an exception.... so that is okay.
Does your HUSBAND usually go to your son's games too???
If not, maybe he doesn't want to, and wants to do his own thing... and does not want you to go out with your friend.... because of that.
I have seen Husbands do this. Expecting the wife to go to kid functions... so they can do what they want.
Your son... will not be traumatized by your not going. Just explain to him... I am sure, he can understand.
I think this is more an issue with your Husband... not your son.
I think, your Husband... does not want you to be gone ALL day... for hours... and then he has to go to the game and do all the other stuff, that a "Mom" usually does...
All the best,