How do I respond to his break up text?

First, I'll say that breaking with a text message it's horrendous, I think it is something too important and that it deserves time for explanations and for example, if done by mobile, you can misinterpret things and hurt the other person unintentionally. Answering the question, if he/she sends a message to break with you, I think it would be good to explain how you feel and what you think without hesitation, directly, and if you wish, ask to meet in person and talk more seriously about the subject.

Receiving a break up message over a text can be both shocking and overwhelming, especially when there is not obvious sign on that coming. The first thing to do is taking a few deep breaths before responding to that because if we act out based on our fear or anger or sadness, often times we end up saying things we regret. After you feel a bit more calm, you could try to perhaps text back saying you understood that something went wrong in the relationship but you are not quite certain to why (in case you really don't know) and asking politely if you two could meet up for a face-to-face talk, so you could understand what is going on. Remember to always be calm and polite in your messages, if the person refuses to meet the time you wish, be respectful and accept that. Give the person the time he/she needs as confronting can be difficult for them as well. If you manage to get to talk in person, be careful not to blame the other person, try to listen carefully what made them decide that way. If the person is certain about the decision to break up, respect that and tell yourself that this issue is related to some behaviours that are not acceptable, not you as a person, don't take it as an attack from the other person.