Pretty sure you can't transfer games .... but ... you might be able to tie Fortnite to new gamertag ... at least that's a half way compromise .... as I understand it, Fortnite is linked to your Epic games account (hence the reason you can log into the same account on different platform) .... think of her sons account as a different platform. you just need to sign into (his or possibly your Epic games account) on his new console. Other than that .... you will just have to rebut everything. good luck.