I'm assuming both units were registered with unique email addresses as you say. That has forever (well, forever unless you call TomTom tech support) associated those two units with those two email addresses. You do NOT want to re-use either of those addresses with the new unit without some account gymnastics, or you'll be on the phone with tech support trying to straighten out your lifetime map subscription for your 2505. Also, your son can't just use a new email address with one of your older units. Either he must use the original address you assigned to it, or the address and password need to be changed by you to suit his preferences.
Tech support can sort this, or you can TRY to use the web site to modify the email address for one of your units to something your son will use. Since you own the email and password, only you can make that change (or tech support, if it comes to that). It can be done on TomTom's web site (Connect unit and let Home do its thing. Meanwhile, go to
www.tomtom.com - Make sure you're logged in for the correct unit email address - Support - My account details - change email and password using links in upper right corner.)
However, I'm not convinced that TomTom's server will correctly 'forget' that email address for the original account even after you've changed the account for the one old unit to something new. If it remembers it for your old unit, you've got issues that will require that phone call.
Cleanest possible solution: Provide your son with email and password for the old unit you will give him and let him use this to update the old unit as he sees fit. Use a new (unused) email address for your 2505 when it comes.