If he got a CCJ at the wrong address, and therefore didn't see the paperwork, he can apply for a "set aside", which restarts the court process. IF and only IF he has a reasonable prospect of a defence.
This sounds like a dangerous course of action to me based on what has been said because ....
My 22 yr old son is being chased for some supposed debt he picked up at university a few years ago , he's had a few letters sent , but to the wrong address , our "helpful" postie has recognised our surname and has kindly re-directed these letters to our house . Am I right in thinking all the paperwork has to be exactly right with this sort of thing ? to make it worse we think that the solicitors have sent snide paperwork pretending to be a CCJ , is there any way I can check with the courts if it's good paperwork ? I've been one site but they want £10 too check it out .
So depending what paperwork was received, claiming it wasn't served could be perjury

. Also, if the address is unknown to the debtor, it is valid to serve papers "at the last known address".
The CCJ in and of itself isn't a problem - it's just the courts ordering him to pay up. However, he only has a fixed amount of time to pay (28 days IIRC) and after that the claimant can apply to the courts for a default, and Baliffs to be appointed - it's at that point the credit file gets hammered, and the fees rapidly escalate.
Given that the son seems to have ignored everything so far, and getting the CCJ appeal wrong could be very expensive, I'd say just pay up now.
sadly my son , who hasn't got a penny to scratch his a**e with
If Baliffs get appointed, it's very likely they will try to scratch his a**e with something a lot more costly and blunt than a penny.