So keep it simple and factual, no speculation.
If what your son received is a N1SDT form, it contains the Particulars Of Claim, and the defence is that they are false.
State, as in your first post, that your son was neither the driver nor the registered keeper of the car (the PoC probably waffles that he’s being pursued as either of them). Personally I’d leave it at that, but others may have alternative suggestions. But don’t talk about previously having driven the car, don’t talk about the possibility of having been nominated by the ex-wife. Think about anything which shows his presence elsewhere at the alleged date/time, but don’t put this in the defence.
Oh yes, probably appropriate also to state that he’s received nothing from the claimant, such as a PCN, so has not had a chance to contest it before now.