Congratulations on your little boy! Scans can be deceptive; errors are so common that the place where I go for ultrasound testing, as a matter of policy, will not give out opinions on the sex of the baby. Even experts can make mistakes! Now, with any luck, "he" won't turn out to be a "she"!!
I really do hope things work out with your marriage, but I agree with the others who have said not to make any big decisions at this time. You have said something really important - "I still believe we still basically love each other". That's key.
I know that every marriage is different, but I would like to tell you my story just to show that serious conflicts can be worked through. My husband and I went through a HUGE conflict that came to the surface 4 years ago. I had told him right at the beginning, when we were first going out in our early twenties, that I didn't want any children and was not likely to change my mind. He was fine with this and we went ahead with our relationship, got married, and embarked on what seemed like a pretty fun life. Well, as time went on, he was no longer so fine with things anymore, but we didn't talk about it. We started growing apart, probably just 2 years into our marriage, but I wasn't aware of it at the time. I naively thought I was happy, and ignored the insecurities I sometimes felt (I used to have a lot of dreams about my whole life falling apart without warning). I had my own issues that I didn't discuss with him (including the fact that I had almost had an affair). Anyway, 4 years ago in August, he had seemed really ill (stomach problems, couldn't eat) for about a week, and I asked him what was wrong. After many minutes of saying "nothing", he finally confessed that ... (a) he wanted kids; (b) he wanted kids more than he wanted to stay with me; (c) he thought he was in love with someone else (who, incidentally, did want kids). This was devastating for me, and a complete shock -- not to mention right out of the blue. He said he wasn't going to pack up and leave right away, but we would have to begin to sort things through. I asked him to go to counselling with me. It took awhile, but in December 1998 we started seeing a marriage counsellor. We spent the next 2 years visiting this psychologist 1, 2 or 3 times a week (depending on our crisis levels) and we both spent a lot of time crying our eyes out. It was frightening and it really did feel hopeless a lot of the time. We talked about all kinds of things I would rather have kept buried, such as why I didn't want children and why I was not comfortable being honest with my husband. I had to hear him say a lot of things I didn't want to hear either (you know, like hearing him say he wanted someone else). As horrible as the experience was for the first few months, we both learned a lot about ourselves and each other, gained new respect for each other's strength, and we remembered why we had fallen in love in the first place. And, yes, I changed my mind about the kids thing. I came to realize that if I didn't try to have a baby, I would come to regret it for the rest of my life. So here I am, 36 1/2 weeks pregnant with my first child at 41 years old, and I have to tell you that I have never been happier in my life. This was the right decision for me (the whole thing: staying with my DH, learning to be open with him again, agreeing to try for a kid). My DH is my best friend again, and I can't even tell you how excited he is about our baby -- it is so wonderful to see. So, I guess the moral of that story, long winded though it is, is that I was in a conflict that seemed to be unresolvable, but it did get resolved. I am a different person from what I was four years ago; I'm much more confident, and less likely to cover up my own needs in favour of someone else's. I pray that you will be able to heal your marriage too.