Hello Joe Plazo,
Welcome to Microsoft Q&A forum. I'm happy to help.
I understand that you would want to download and let your son play the Ultimate Edition of Forza Horizon 4. The bundle itself has been delisted, so unfortunately new purchase is not available. The account that previously bought the bundle should still hold license of it though. In this case, you can try to log in to the Xbox app using the account that made the purchase and check the library. If the game was bought on the account you should be able to see the game and install it from there. However, please note that only the account that made the purchase can enjoy the benefit of the bundle, which means you can only see the bundle content logging in with your account. I hope this clarifies.
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