She's sneaking around with her ex-husband and hiding it from her son
Not to sound jaded, but I can think of more scandalous scandals.
You're casually dating your ex. OK then. You made the choice, now pick your preferred consequence: letting your son walk in on this news, or telling him yourself.
Not that it's mine to make, but the choice seems obvious. "Just so you know, your father stays over here occasionally. I didn't want you to find out by walking in on him one day." When he asks you what's going on, you say, "Nothing serious, we're just friends." Which it isn't, and you are. You don't have to explain yourself, not even to your son.
The getting-back-together prospect might have meant a lot 12 years ago, but surely now Dave can handle more nuance.
He may be concerned about your making things difficult for yourself, and he may be right, but you would be right to assure him he's not responsible for your choices, you are.
And finally: It'll be weird for him, but only if he doesn't give it more than a moment's thought. You were drawn to each other once, so why not twice?
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