I don't think it's giving him "a say" by letting him know. I would hint (to him) at the possibility of your guys getting married, followed by telling him you guys want to get married, will get married, and so on until it doesn't come as a shock to him but never make it an official announcement that he would either have to hide or disclose from his BM. Then when it's done, SS isn't in shock and BM has missed her chance to throw a fit. (LOL...until she does)

You might think of Dad and you telling the child as before you and Dad go off to elope if you think it's important for the child to know before. But do not ask the kid what he thinks of it. It would be nothing more than the night before leaving 'hey, sweetie, Coco and I are eloping and getting married this weekend, we'll see you as usual after we get back home'.

Who cares if he tells BM. BM can have her meltdown over by the time you get back.

He doesn't have to keep any secret from child , just tell him last minute (too late, you're on your way, lol) and he'll will know when you get back that Dad and you are DH and wife and he now has a SM. Good, bad, indifferent. He gets you.

It isn't the child's decision. Don't think this kiddo will come and ask Daddy's permission before the kid, himself, one day picks a wife and marries.

You can tell the kid just as you two are about to leave or not, it's not going to change the marriage plans.

Congratulations on the upcoming marriage.

“I think he needs to have some warning, and be able to voice his concerns and feelings.”

Nope, nope, nope. Eloping doesn’t need to turn into Child Psychology 101, because it’s not going to “work well” for a 10 year old boy.

You go on to say: “But, I do not want him to be a part of the ceremony, I want that to just be my fiancé and I.” That’s like rubbing someone’s nose in something stinky. Do you really feel it’s necessary to tell him, and only him, still?

“I also don't want to tell him that it's a secret, because I don't think kids should be made to keep secrets. I'm sure once BM finds out she might go off the deep end...”

Read that sentence again. You’re contradicting yourself here. Do you think a 10 year old should, and will, keep this a secret from their own mom? It’s almost as if you DO want her to find out. Have some wits about it and set a good example while avoiding pre-elopement drama - keep your elopement under wraps until you two return and it is a done deal. Why in the world would you want to leave a wake of emotions and drama, as you drive, fly, whatever... away?

Your post reads a tad vindictive, imho. You have the child 50% of the time. It cannot be that hard to avoid talking about it around him.

Elope. Come back, tell ss then do something special with the 3 of you to help celebrate your marriage as a family unit. That way BM won’t know, ss is not out in an awkward position and he still gets to do something to mark the change in his home and celebrate your family. If you make the special family celebration something tailored to the three of you then he will appreciate it much more than a boring wedding ceremony.