V Bucks Microsoft Account transferring to Fortnite game?

In a word: no.

The V-Bucks you've purchased may have been redeemed against a Microsoft Account, but they've actually been given to a linked Epic Games account for Fortnite. Epic Games decided to disallow the transfer of V-Bucks between accounts to prevent kids (and adults, I guess) from falling victim to scams.

- Jason

They will be able to play any games you have downloaded but skins and V-bucks in fortnite can not be transfered from your account to theirs. So ensure if they want a skin or V-bucks, you buy it on their profiles.

It say in the Description V-Bucks you purchase from the Microsoft Store can be used on Xbox, PC or mobile, but can't be transferred to or used on other consoles. https://www.xbox.com/en-au/games/store/fortnite-5-000-v-bucks/c20fm0b4q9kc?SilentAuth=1&wa=wsignin1.0

Did you buy the V-bucks on PC version of Fortnite or Xbox?

If you we logged into Fortnite on PC, you have to wait a few seconds before the V-Bucks show up. The V-bucks transfer from PC to Xbox after you bought them and they were accredited to your Epic account. If they don't show up you need to contact Epic games
to review your purchase history. If you bought it on Xbox you can do that at account.microsoft.com > payment and billing > purchase history

Consumables like Fortnite V-Bucks have to be purchased by the account that is supposed to use them.

Unfortunately, there is no way to transfer consumables to a different account. While it is possible to buy a game and some DLCs that your son can play thanks to the Home Console setting that shares your game purchases with the first console you set up with
your account, consumables can't be shared to a console.

Therefore your son can't see the V-Bucks in his account, as the V-Bucks are stored on your account, and not shared to the console.

In order to get your son V-Bucks, you would have to buy him a currency gift card, that your son then redeems, and with which he can then buy V-Bucks for himself.