As an intern, part of what you are supposed to be learning is how the professional work world works. One thing you should understand is that it is almost never in your best interests to have nothing to do and not let your boss know. This ia sign of passivity and is not looked on as a good trait.
If you are finished what you were given to do, go to your boss and ask for more work. Create a list of things you would like to do in your free time if you have any and then suggest one of those things if he has nothing immediately for you to do. Volunteer to do tasks no one else wants to do. Ask to shadow another developer and work at pair programming with him. Ask your boss directly what you should learn to make you more useful to him.
If he never has anything for you to do, that is a huge red flag. For an Intern it means one of several things, either he doesn't think an intern in general can handle the projects he does have or he doesn't think anyone has the time to get an intern up to speed or he has given up on you in particular. If regular employees are working lots of hours and you are not, then you need to be especially wary as he clearly has work. Ask him if he has a problem with your work.