Ditto on everything Howard said.
I would not consider a rising of anger as something to avoid or something that went wrong. I would view it as great material to work with and to begin to crack the egg, the hard shell of his emotions. My suspicion is that weakness and vulnerability led to lack of safety in his experience – perhaps, he was ridiculed for his empathy, made fun of, got in trouble in some way, mocked or beat up. I wonder if he can remember a time when he had a similar feeling in the past. Sometimes abusers will look for a persons weakness or try to elicit it and then pounce with they see it – like a tiger seeing blood. Next time you see him, it might be helpful to revisit the situation, see if anger arises again, ask him where he feels that anger in his body, ask him if it has a voice, what it wants to say, what it wants to do, etc.
I thought you did a great job.