Would like to transfer Microsoft balance from my son's account to mine. How do I do that?

As an Adult account in a
Microsoft Account family, you can add funds to a child's account, but that doesn't work vice versa. The process to add funds to

their account can be found here.

But you won't be able to add funds from your child's account to the Adult, those controls just aren't in place.

It sounds like the Privacy and Online Safety settings might be what's preventing him from purchasing
the title, so if he's in your Microsoft Account Family, you should be able to set permissions as you are comfortable with to allow him to purchase the title.

If he's not in your Microsoft Account Family, then I would recommend reviewing this page here.

Please let me know if you have any questions about what I mentioned, or run into any errors.

Buy it and gift it. No other way. Thank's for being great community member. It's a family thing I see. Great!

you should also trying contacting xbox support, maybe they can transfer it, although i wouldnt bet on it

is he unable to buy the game because of an age restriction on his account?

the simplest method would be for you to change the restriction on his account and then he should be able to purchase the game.

The one thing that was not answered for you and this is assuming that the game is a 360 backwards compatible game and you are trying to do this on a xbox one is this.

The ONLY way to use MS account currency to purchase anything backwards compatible would be to do it directly on a xbox 360 itself you can not use MS account currency to purchase backwards compatible items on a xbox one or on the MS store online.