Buying Foreverton membership won't help as you usually need a purchase history for Derby tickets.Your options are going to be the following - in no particular order of sense or sanity:
1) buy a Foreverton Membership for £30, for both yourself and your son. I'm not too sure here whether the Derby game requires this PLUS attendance at previous home games in the season but the Foreverton will give you priority of 3 to 5 days before the General Sale. The majority of tickets will go extremely fast on the morning - so it's best to be on the site at 8am when they go live. The cost here altogether for the Foreverton Memberships plus tickets will be £150 max.
You'll need to make sure you buy a membership EACH, and then link them online under "friends and family" in order to be able to check out.
Aye I thought that was the case, potentially could change this year with the membership changing and the continued season ticket sell out.You don't need a purchase history for home derby tickets. You just have to be quick.