That's really hard to solve. I mean, I can understand not wanting to talk about something when being upset (I am the same way) but he can at least work towards stating that he has something on his mind and he just needs some time to sort through it and will be open to discussing it if it becomes necessary. Now, later on, he may have sorted it out and realized it didn't need to be discussed because it was truly a reflection of self (which is pretty much what our emotional reactions are). Our negative emotions are normally triggered by a lack of communication or an engrained memory linked to that emotion-both of which can be discussed.
Ever think about telling him that all relationships but, especially, one of this nature must have clear, transparent, and honest communication to survive and you are interested in working towards survival?