This is just my opinion and pray, what business is it of mine? but, I think if I had a spouse who thought of me as a sibling, withheld intimacy from me, and was no more than "cordial",
I would leave. I wouldn't ask anyone. I would leave. Take your son with you.
You see, a prostitute can give sex but you need much more than that. You need LOVE. Touching, affection.
A mistress might give both but it's only temporal.
Saying this and reading what you have written, I don't think you will leave. You will probably stay and a year from now, five years from now, you will be on a forum asking the same question.
Sorry, but that's my impression. I know you're in a miserable situation but you have to have the "gumption" to leave.