APHA horses are a breed. Saying they are a mix is just asinine. They have limited out crossing and have breeding requirements.
Just FYI, the tobiano pattern has never been nor will it ever be in AQHA or JC horses. Tobiano's were one of the first types of color patterns that were accepted by the APHA.
Harley - Your horse is registered as a Paint, so he is a Paint. Regardless of what may be way back in his breeding, his papers state him as belonging to the APHA. So that is what he is.