My boyfriend has wanted to go to the army for a very long time and when him and I started getting serious he's having second thoughts because he thinks its not fair for me if he goes. I'm trying to convince him that its okay and ill support him 100%. I don't want to be the reason for holding him back from doing something he has a passion for and I have tried telling him all this but it just isn't working. Does anybody have any advice for what I should do?

We don't see the whole picture. Does he have a job, is he in school? Then let him have second thoughts. If he's aimless and out of work, he may be thinking (rightly) that the military will help him find his path. He may also be using you as his crutch to keep from making ANY decisions, so try to tell us more. He may be worried about losing you (does happen of course, when apart). He may even be worried that you are too eager to see him join.
In other words, no matter what reason and rational thinking you use, he won't accept it if it isn't what he wants to hear. He has to decide.
One thing I might do is ask him about all the branches, what they offer, what he'd learn, where he might be sent, what benefits he'd have when he got out, and so on.
Marriage of course would solve some of this, but you don't say how far along you are with this.

There is an unfortunate reputation of infidelity between soldiers and significant others when deployed.

No one knows the situation better than the two of you and there are a lot of factors that must be considered before making a commitment to serve one's country. There is an equal (or greater) commitment that you must make to each other.

I was single for my deployment so I'm not the best source. I know plenty of soldiers that cheated or were cheated on while deployed. It's also one of the leading causes of suicide and reckless behavior.

Sorry I can't be more inspiring. Everyone is different.