Me and my long distance boyfriend had an argument and he told me to let him be

That was pretty short and sweet. A little more information is needed, however if he told you to let him be, then I would do exactly that. Sometimes people just need to calm down after getting worked up. Don't know any of the details behind this however, so any clarification you have would be nice.

More information is definitely needed to this story for us to help you properly

Do what he asked, let him be.