There seems to be a MUCH bigger issue here than "should our child go to private school."
Please re-read your own post, as if you were reading a stranger's post. Try to stand back a bit and be as objective as you can.
There are a lot of red flags here, and again, they are not about school:
"Since my husband and I were arguing already...."
"I don't even know where he wants to send him.....
"I am really uncomfortable with all these things my hubby has sprung on me..."
"We have our intruder who has already left to be at our vacation spot...."
You and your husband are not communicating. Period. Doesn't he realize your son is NOW doing OK with school and loves it, or is your husband's memory of school stuck in 4K?
Why were you "arguing already"? You don't say but it sounds as if it wasn't about school since he "sprung" this on you. And one rule of fair arguing is not dragging in issues that are unrelated to the thing you're talking about. From what you wrote here, hubby dragged in the school issue out of the blue, apparently unrelated to whatever else you were arguing about, and blindsided you with it. Not done -- and it puts your son's schooling into an argument that (I assume) was not about that at all.
You don't even know what school (if any) your husband has in mind. I'd bet he has none in mind and dragged the issue in either to distract you from other topics (whatever those are, that are making you argue) or to needle you and give you something else to be upset about.
You need to separate the school question from whatever your other issues together may be. I'm gathering from reading between the lines that there ARE other issues (and I have no idea who the intruder is, but that may be part of the mix?). Have you told your husband what you wrote here? That your son is happy, loves his school, expects to see his friends next year? Unless your husband is making an argument that the academics are a problem, or there is bullying etc. -- and it sounds like he isn't -- he seems to have zero reason to bring this up UNLESS....
He's trying to hurt and upset you.
You and your husband need to agree that school and your son are not issues for discussion until other issues between you are resolved. And it sounds like you may need a third party to help. I'd get a marriage counselor involved and say you need communications help -- together. If hubby won't go, you have an even bigger problem, unfortunately.