I was in your exact position OP.
I had no preference for our first baby, which was a boy, but when it came to Baby number 2, I knew it would be our last baby and so I was hoping for a girl.
It wasn't because I specifically didn't want a boy, but because I knew that it would be my last baby and that if it wasn't a girl I would never get to experience what it would be like to have a daughter.
Anyhow, the 20 week scan came round and we were told it was a boy and I did really sad. I tried to act all happy and smiley to match the energy of the sonographer and my husband but inside I felt really sad.
It took me a few days to accept the fact that I wasn't going to have a daughter but once I made peace with that, I then started to feel so excited at the thought of having another boy!!!!
As I'm sure you're aware from your current son, boys are amazing, and having two of them is incredible!
And when I look back on those day of finding out the sex, I did want a girl for my own sake, but for my son's sake I am so glad it was another boy. My personal opinion is that in general, siblings form closer bonds with siblings of the same sex, than they do if their sibling is the opposite sex. I know that is just an anecdotal generalisation, but it's still one that I hold.
My boys are 11.5 years old and 8 years old now and they absolutely adore each other!! They love each other so much, they do everything together, they are always telling each other, "you're my best friend" and they just can't be separated. Even at the age they are now, they still don't want their own bedrooms and love sharing. They do the same activities, they share the same hobbies, they have a lot of joint friends, they share a lot of interests, they spend a lot of time with each other outside of school, and I don't think this would be the case as much if they weren't the same sex (especially as they are getting older now).
They have got such similar physical qualities too - they're little carbon copies of each other. They're just adorable!!!
To summarise, I am so, so, so unbelievably glad that I have two boys, watching them grow up together and form such a close brotherly relationship has been so special and I hope it is one that continues to thrive.
If my husband ever agreed to a 3rd child (he won't) I would absolutely want another boy to add to the gang!!! My dream would is to be surrounded by lots of lovely sons!!
If you do find out you're carrying another boy, allow yourself to grieve for the daughter you'll never have, but then shift your focus to the wonderful son that's going to join the family and start imagining how wonderful that's going to be, not just for the joy he's going to bring to your life, but also because it means your current son is going to get to grow up with a brother - and then smile as you think about how that could be a lifelong, really special relationship for them both.