I think the statistic is 1 in 32 men, so... they're everywhere. How will he be monitored? Erm... he probably won't be. There's that many of them, they can't watch them all.
He'll be on the sex offenders register for a specified amount of time. He has to notify the police if he changes address. He'll have to surrender his passport OR tell the cops if he's planning a foreign holiday. He'll have to let the cops know if he starts seeing a woman with kids, that kind of thing. But that's probably about it.
He may (will probably have) have a sexual harm prevention order imposed by the courts, which means 'conditions' can be imposed upon his life. For instance, he may be ordered to stay away from schools, or parks, or the local arcade, depending on his offending pattern. The SHPO may impose conditions on his computer and phone use, for instance, and he may or may not have to tell them his phone numbers, or let them look at his laptop if they visit him, or never delete his browsing history, or not be able to have TOR or use encrypted comms channels, but it depends on the conditions of his SHPO. Do the cops check? It depends. Some police teams monitor them better than others. They can call at an offenders house and do spot checks if they want to, but there's no cast-iron way to check he's doing as he's told. Can he have six phones and seven laptops nobody knows about? Sure. Unless they boot his door down and search his house thoroughly, (and his Mums house, and his best friends house, and maybe his work place) they'll never know. Will he have a probation officer, or somebody calling on him at home regularly to check he's being 'good'? Highly unlikely. Not unless he's considered a 'serious' high risk, and a few kiddy pics on his comp is just the thin end of a very ugly dirty wedge. If he's got previous for dom V offences, 'contact' offences (has actually touched kids with his own hands), etc, he can be moved up the risk scale, but he's probably deemed 'low-risk' and not being monitored too closely. That's just how it is. If he's found in breach of his SHPO he can be recalled to jail, but they have to catch him at it, first, and that isn't so easy if he doesn't want to get caught.
sorry state of affairs, ain't it?