I mean, unless he's messaging the girls, isn't it the equivalent of viewing porn? I don't know what your policy on porn is, but watching it and masturbation in general seems normal to me.
I don't know if following those accounts constitutes as "lusting" after the women, and the fact that it is public likely means he didn't even consider that this would bother you.
If it really bothers you, talk to him in a reasonable manner before dumping him. The conversation may lead to a breakup, which is also fine, but you could give the guy the chance to say his peace. I think you should be aware, though, that even if he agrees to not follow the women that most of the profiles that men follow on Instagram of half-naked women are public. Oh, and he definitely watches porn, so it really won't change anything at all. The fact that you say you want him to hide the behavior from you strikes me as weird. Don't you want to know who your partner is and that they are being honest with you?