Abasan

1. Tell him its great he was able to recognise the situation and control himself.
2. Tell him everyone is different and you'll meet saints and jerks along the way.
3. Tell him doing it in the dojo, is one step to doing it outside.
4. The purpose of aikido is to find harmony of one self and others regardless if they are saints or jerks, in the dojo or outside.
5. Therefore practising it with this purpose in mind will help him achieve the control he needs.

Personally I think loosing it in the dojo is unacceptable. Since you've established the reason for training the moment you step in. But going any further along this line will sound preachy. So hope you manage to convince him.