Wow, uh, it sounds like his is one of the few that gets along with his ex. This is NOT odd - it does happen, usually to couples who are both genuinely nice people
who have realised that they just grew apart / out of love.
There does not seem to be a 'WORST' of it - he was open with you about he text him, and what it was about. So he was not hiding anything. Even his going outside is considerate of you, because he knew that the text message upset you.
You seem to have insecurity / trust issues.
The easiest way to know if someone is trustworthy is to see whether or not their actions match their words. Trustworthy people are consistent in word and deed. The only qualification I would put on that when working out whether or not his words match his actions, you must look at it from his point of view (ie. you cannot use your past as a judgement on whether or not he is consistent - you must look at what he says, understand it, and see if it matches what he does)
I mention this, because it can help you overcome your trust issues, to truly understand your bf (though if by chance, he is a liar, then you will work that out too)