The law changed on 10th March 2014. Any conviction resulting in a prison sentence of 6 months or less now becomes 'spent' two years after the end of the sentence. If the sentence your partner's son received was one of 12 months (resulting in him actually spending 6 months in prison), the conviction will have become 'spent' four years after the end of his sentence.
Therefore if he's asked (either on an application form or in an interview) whether he's ever been convicted of a criminal offence he can lawfully answer 'No'.
The law also relates to 'ancillary' matters. If, for example, your partner's son had been refused some kind of insurance (before his conviction became 'spent') solely because of his criminal conviction, then he can now lawfully answer 'No' to a question asking "Have you ever been refused insurance?".
There are just a couple of important exceptions to the basic rules:
1. No conviction can ever become 'spent' while the offender has another 'unspent' conviction. So if your partner's son had another conviction (which was still 'unspent' at the time that his first one would normally have become 'spent') then his original conviction wouldn't become 'spent' until the same time as the second one did.
2. Certain occupations and activities (such as working with children, vulnerable adults or 'in the administration of justice) are exempt from the rules and, when applying for such posts, convictions are never regarded as 'spent'. [There's a small exception to that statement but it's probably not relevant here anyway].
So, unless your partner's son is applying to work in one of the few 'exempted' sectors (such as working with children), and assuming that he's not got another 'unspent' conviction (which is delaying the first one from becoming spent) he can lawfully answer 'No' to any question about whether he's ever been convicted of a criminal offence (or to 'ancillary' questions, such as "Have you ever been to prison?").
http://hub.unlock.org.uk/knowledgebase/spent-now-brief-guide-changes-roa/