If he's telling you the truth, then his claim is against his registrar, weakened somewhat by his lack of attention to the address and/or contact details matter(s).
Worried about a DRS? I wouldn't be, as the fool has given you all the info you need to demonstrate that the fault is elsewhere.
Interesting that Nominet recommended he contact you directly. Maybe they don't think he has any hope of getting it back!
I'd put something in writing to him, just to put you side of things on record BEFORE he thinks about DRS'ing.
Of course, don't let this stop you transferring it to your wife/partner/a.n.other like you were intending to do all the way along.
