He wouldn't necessarily be on the sex offenders register for life - it depends on the offence and the sentence - someone is only on the register for life if the sentence is over a certain length (I think it's 30 or 36 months), and then there is a kind of tapering inclusion length of time,and it is also is slightly different again if someone was under 18 when the offence was committed - and this is assuming that any offence happened after 1997 as the register is not retrospective.
It possibly is worth talking to the police, but I don't think that they will tell you anyone (data protection act and confidentiality) and won't confirm if someone is on the list or not.
Do you not know anyone, other than his ex, who has known him for a while - although I understand really not easy to ask subtly!