I have just brought V Bucks - no code?

if you purchased on his account or gifted them to him then yes, otherwise they are on your account as vbuck are account specific

I have just brought v bucks for my son and my Microsoft account but it's not bringing up a redeemable code - how long should I leave it? His xbox is not linked to my Microsoft account.

If you purchased the vbucks from the ingame store or from within the store on Xbox then the vbucks are automatically applied to the account that they were purchased from. You dont get a code.

If you bought the Vbucks from the xbox store it will Automatically transfer to your Fortnite account

I would have him load up the game and check his V Bucks in the game. It should credit it right to his account and should show there.

I checked everything and nothing has been brought with the v-bucks, can you send v-bucks to other accounts?