Does your 16 year old know yet ?
I have no personal experience as a spouse. But as a 16 year old, albeit without the extra added "complication" of a secret half-sibling, I have personal experience by the decade load.
If your son is sitting his GCSEs from May, I think his interests would be best served by the school's pastoral staff being made aware of the sudden and dramatic change in his family's composition. Things like this can, and do, torpedo a child's educational attainment. Not just for the first set of exams they are about to sit, but all the previous expectations can go flying out of the window. When your own parent fails the "be responsible" ruling that teenagers feel has been stuffed down their throat for their own behavoir for years, flying of the rails to some extent is not unknown. Ploughing the first set of exams can lead to throwing in the towel on former ambitions. In a whirl of self destructive "fuck you, why should I toe the line when you clearly don't" to the parent who has show feet so clay like they are practically an asset to the local pottery. Not every kid finds their way back to education. Sometimes the first walk away in a haze of anger and pain can leave them under qualified and stuck at a disadvantage long term. It is worth doing anything you can to head that off at the pass. School in the know may be helpful in that regard.
I know that it must be extremely discomforting, painful and unattractive to have to tell complete strangers the details of an enormous betrayal. But as a family secret it has a terrible potential to rot your son's life chances before he has even begun to create them.
Pleanty of people will tell you he is "nearly an adult", that his parents' marriage is "none of his business". Some children will feel like that. Others will not. With knobs on. And may be left to contend with not only deep hurt, a profound sense of loss for the parent who turned out not to be who they thought he was and ... rage. Lots and lots of rage. For that reason I think having a an idea of how to access to age appropriate counselling before very clear and overwhelming symptoms of "not coping well" are on display, is a good idea. Again, school may be able to point you in the right direction on that one. They may even end up being the safe place he can go and unload a teetering pile of feelings he doesn't know what to do with.
I am so so sorry. For the enormous betrayal you are having to cope with, and the pain this could cause your son.