There are a few different things going on here.
"...worst case scenario should be a 6 month custodial sentence, so if the worst happens would he be on the register for 7 years as he is an adult"
This is where things start to get complicated.
To start off, there is no such thing as a "Sex Offenders Register" in the UK.
This is simply the common term used for an offender who is subject to the "Notification Requirements" under the Sexual Offences Act 2003 and who will also likely be subject to a SHPO (Sexual Harm Prevention Order) or, from years ago, a SOPO (Sexual Offences Prevention Order).
"...a 6 month custodial sentence"
Normally, if a person were sentenced to six months then, under the Rehabilitation of Offenders Act 1974 (ROA) the offence would be "spent" 12 months after the custodial period ends (so, 18 months after they go into prison, even though they will be released at the half way point - or even the 40% point these days).
A "spent" conviction does not normally need to be declared - unless you are applying for specific jobs, like working with children or vulnerable adults or in roles like a police officer or prison worker etc.
However, spent convictions are very different when it comes to sex offenders. This is because most sex offenders are also given a SHPO (previously SOPO) which can be for any length of time, but is generally the same length of time as the notification requirements.
So what this means is that an offender sentenced to 6 months in prison, instead of having their offence being "spent" after 18 months does not have their sentence "spent" until the SHPO ends.
The reason for this is that a SHPO is a "Relevant Order" under the ROA and so an offence is not "spent" until the SHPO comes to an end.
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So, the practical upshot of this, is that, if he is convicted then he will have to declare this as an unspent conviction for however long the SHPO lasts for. If he is sentenced to 6 months then the SHPO will likely last for seven years.
If he is sentenced to a community order then any SHPO is likely to last for five years or a conditional discharge would last for the period of the discharge.
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So what does a SHPO consist of?
This all depends on the nature of the offence and it is aimed at preventing the specific type of offence that he has been found guilty of being repeated.
For example, if it's something like the recent case of the BBC broadcaster Huw Edwards, then there would be rules on not deleting his internet usage and making it available to police officers when requested.
If it was a contact offence then there would be rules around contact with children. This may be limited to the gender of the children that he committed the offences against, or it may include all children if appropriate.
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"But I need to know what I'm facing by association."
Everything turns on the specific facts of the case.
One thing I will say though, if you have children of your own, then you will likely come under scrutiny yourself to determine if you can keep them safe from him.