Question about handjobs

I am waiting for sex and had never touched a penis before my current boyfriend. I am only comfortable touching it over his boxers so far and the last time I did I was able to feel it throb a few times but when it got a bit more frequent he stopped me. Could he just not want to cum with his clothes on? He said he won't go take care of it when I offered it to him in case our make out sessions got too intense. What can be the reasons he is stopping me?