Unless they have changed their rules, Disneyland does not allow outside food or drink into the park. That being the case, if someone at the park did not serve it to him, he could not have imbibed it there unless he smuggled it in. He could have entered the park drunk, I suppose and if he seemed calm enough I suppose no one would notice.
They wouldn't let me bring a gyro into the park and they don't even serve gyros there. I'm sure they would be even more stringent when it comes to alcohol. Based on my experience, therefore, it is most likely that someone at the park, or many people in different places served him enough alcohol to put him over his limit.