Congratulations!
I think that 12 is still a fairly self centered time and he is probably worried about how this will affect HIM and HIS life. I think though, at 12, he is old enough for you to address it with him directly.
"If I were you, I would probably be freaking out that mom and dad were having a new baby and worried that we wouldnt have enough time for basketball practice anymore..." etc etc.
I think that if you validate his feelings without giving them more credit than they should have (meaning its not going to change your minds about adopting) he will come through it fine. This is the same reaction that alot of 12 year olds have when their parents are having a bio baby (except there's that gross my parents had sex factor to deal with
).
Let us know how it goes!
Jen