No matter what his motives, don't let him mess with your thinking! We learn a lot in group therapy about cognitive therapy... changing the way we think for the benefit of our health, and it is true that we can work on changing the way we think into more positive ways of thinking that will ultimitely improve our health. But on the same hand we don't want to let other people influence our thinking in a negative way... case in point your ex's comments about your bf.

The ONLY think that matters is are you upset or think it is wrong for him to go on the trip without you? I doesn't sound like there is anything wrong with it, he is just going to do something he enjoys. Now your ex is trying to tell you that there is something inherently "wrong" in that behaviour... don't let him put those ideas into your head! The only "right" or "wrong" are what you decide is right for your relationship. Don't let him define a "ruler" for you to judge your current bf against, especially when that ruler is all gonna be marked with inchpoint that show you bf in a bad light and your ex in a good light.

Instead of your ex worrying about "what your bf would do if he really cared" what your ex should be thinking about instead is what he himself should be doing if HE really cares about you. If your ex really cares then he should NOT be trying to manipulate you in your relationship with your bf... that is a sign of disrespect and selfishness, not of caring and love.