Eh, it's just his preference. I think this is a good opportunity for an interesting conversation. Ask him what? When it started? What prompts his continuing to do it? See how he answers. I mean, maybe his mom made him do it when he was little. And it's a habit now. So, chat him up about it. Could be a good time to get to know each other even better!
The part where you say that sometimes you go to touch your boyfriend's penis "to initiate arousal" -- maybe he wants to get an erection with his own timing as erotic events unfold between you, instead of having you cause him to get one.