I know others have differing opinions about this. But:
"You can't get diabetes from eye candy."
One of the conditions in my relationship is: You cannot control how I use my social media. And I will look at anything I want, including adult material online. In return, I won't do the same - control your social media or censor what you can/cannot watch online. I will be discrete though, and do it on my own time and not in front of you.
I also won't talk to any of my exes (done and done) and I won't make friends with any single women - because that would be disrespectful.
Of course, every relationship is different, and if this is the line you want to draw in the sand, that's between you and your BF. So to answer your questions:
The only way to express this boundary is to make it as honest as possible. This is about you and your anxiety, and if he is 'respectful' to your condition, he will stop liking sexy girls on IG. Hopefully, you have reciprocated with respectful things for him in return. And I'm not talking about you unfollowing guys on IG, because I really don't believe he cares what you do with your social media (and if he did, he's hypocrite).
The length of the relationship doesn't matter. If controlling his social media is a dealbreaker for him, might as well find out now and save yourself any more time, because the relationship is over. Of course, he could just lie to you and buy a second device with another IG account for fun follows. Just something to keep in mind.
Right/wrong. You're going to get differing opinions about this. Personally, I think you're wrong. But others may think you're right, this isn't something that people can be universally agreed about. But, imo, if you have problems with him liking girls on IG, his bucks night with no strippers is probably going to be sad and boring.