Dry for 3 months then always reverts back to wet

[Hh19]

Hi there

My 11 yr old Son has been going through this pattern of dry every night for 2-3 months then starts with one wet night and ends up every night . Goes through a spell of maybe a month to get dry again then same pattern all over again . Any ideas or advice ? We use alarms when he’s wet but he sleeps right through them and only realise he’s wet when he does get up . Just don’t understand why this pattern keeps repeating.