First I'm an Avast User not an Avast Team member.
The registration (product) and account are linked to your Avast Account.
So since you have already setup your Avast Account, with a user name and email address and is linked to your current payment (it should still work for your nephew). Given the linkage I mentioned above you should get a reminder prior any renewal and payment would be taken at the renewal date if you have setup the auto renewal payment.
If your nephew's setting up a new Microsoft account doesn't require a fresh install (which I doubt) then the Avast Premium installation should be none the wiser to your generosity
I hope this addresses your concerns.
Points of note:
If your nephew right clicks on the Avast Tray Icon and then Registration he would be able to see some information and be able to access your Avast Account, which you may not want.
See attached images - this comes from my Avast Free installation which will differ from yours, but if you access your installation you will see what he could access.