The only way to share your Ultimate Pass is to set your son's XBox as your account's "Home" console; he can have his own account and can piggy back off of your Pass with all benefits, but only when he is logged into that console, and as long as you aren't actively signed-in on both (ie. make sure you don't auto-sign-in on your son's XBox), you can both play on separate consoles at the same time.
Also, looking at other f2p games on XBox (Warframe, Fortnite, etc.), it looks like Live will be needed. The only reason you didn't need it for PSU was because it had it's own subscription.
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