Originally Posted by
guv1976
Wirelessly posted (BlackBerry: BlackBerry8530/5.0.0.1030 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/417)
"You check in once at the American counter at the first airport. (London)"
No, no, no!
AA does not fly LHR-BOS -- at least, not on its own metal. The OP must check in at the counter of the carrier that actually operates the LHR-BOS flight. If that carrier is BA (which seems the most likely), then BA should be able to issue the BOS-PIT boarding pass.
My error. Was pre coffee where I am and didn't think and misread the post. Edited for OP'S sake.