Goodness, please don't think your issue is taboo here on the adoption threads. We have other birth mothers here too, some who relinquished their child and others who didn't have a choice .
Others are adoptees or adoptive parents . But I think we all understand that life is complicated and peoples circumstances change over time. However you really need to tell your son now, however hard it is. It will be much worse if he hears from someone else .
I'm glad to hear that you have a close relationship with your son, this will help a lot . You need to be prepared that he will be shocked and probably a bit angry . This might be about you and the choices you made but maybe not.
It likely to be about his own feelings. He has a sibling he's never met - how does he feel about that ? Does he have other siblings or half siblings that he knows ? If not, this might have an even bigger impact on him .
He might want to trace his brother / sister - how would you feel about that ? You might not like it but your cant stop him.
Does he know about your relationship with the father of that child ? Did he know him too - was it his step father ? He will ask about him an what happened .
He might ask lots of difficult questions , such as why didn't you give him away too ? Why didn't gran or an aunt keep the baby ? How could you give away His brother or sister ?
He will no doubt ask why you haven't told him before .
Remember he's only 18. If you told your story to a stranger , they would probably be more tactful and avoid asking you these hard questions . But teenagers are not known for their tact, so he might be very blunt and angry .
This will be very hard. And I'm sure you will feel like he is judging you and blaming you . When he's probably not, he's just trying to understand what must have been a complicated situation ,with his very black and white teenaged logic.
Is there someone you could talk this through with in RL, to prepare yourself for telling your son? You need some support to help you deal with his reaction and perhaps strong feelings .
I think that once he has had a chance to come to terms with its he will be fine . But you need to give him some time , after all, it's taken you 16 years to talk about it so he won't he ok with it in a day.
Remember that none of this takes away anything from the love you have for him and the relathioship you've had for his whole life . That's a very solid foundation that can survive a few storms .