If I were to purchase a game on microsoft.com would I be able to load it on an xbox later?

Hello dear Gamer, first it's a pleasure to be able to help you today.

My name is Vinicius.

I am a gamer and independent advisor on the XBOX community platform.

Hey FillerName, how are you today? Hope you're well.

FillerName, yes, you can take advantage of discounts on games and buy them now if you want.

When you buy your console, you will need to connect to the same account you used to buy the games and all of them will be available for download in your library.

If the games were pre-loaded then they were purchased on someone else's account. If they were purchased on your son's account, he would need to set the console as his Home console in order for you to be able to play them as well. https://support.xbox.com/en-US/games/game-setup/my-home-xbox

Yes, any game you buy on the website will be available in your ready-to-install list on the console.

If you have your console in "Instant On" mode, you can even install it remotely (follow the prompt after buying it) and it will already be downloaded by the time you get home. (Depending on your internet speed, of course.)

Thank you so much i was concerned but I am no longer worried... thank you once again appreciated

Forum support can't do anything. You can ask phone or chat support, but there is no obligation on their part to follow through, regardless of your thoughts on the matter. More to the point, if you were able to access the website you were able to "purchase"
the games there, which would add them to your account to be downloaded later.