If I understand correctly, what you are trying to decide is whether to tell your oldest son his mother is not married. Your oldest son is 17. That is a fragile age but if he is developmentally normal, it is close to being an adult. You are concerned that his friends' parents will tell him this information anyways.
If this is a real possibility, then hearing this news from them would be a hurtful and embarrassing thing. That is true. So, before even considering if he will be able to handle the information or not, we have to consider if it is likely that others will tell him. You seem to think so, therefore, it would be better for you to tell him.
The other consideration is your relationship with him. If this will hurt your relationship, then that needs to be taken into consideration and you may need to let the other parents be his source of information about his mother. Because to hurt your relationship with him is not a good outcome of telling him.
So, if your relationship with him is able to withstand your being the one to tell him this, then yes, you should tell him given that he will find out soon from his friends' parents.
Okay, I wish you the very best!
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