I've never had the SSSS so I'm not sure of the procedure other than the extra screening. Is it possible to have a SSSS on a connecting boarding pass? I would think you would only get that on the boarding pass at the first airport since you likely wouldn't go through security at a connecting airport in the US.
I don't travel internationally that often but I would think they would let him get to AMS. If it's not safe to continue on to Birmingham, then the airline wouldn't fly there.