Well you didn't really say what you're protecting it from. If its break-up its a sad fact of life. People grow and evolve, sometimes they grow apart from those who once ment so much to them. It could be fixable though so don't fret. Speak with him about your fears. Hiding from them won't help. Schedule alone time together. No phone, no tv, no one else, and no distractions. Sit and talk. If you have drifted apart it might be awkward at first but slowly you will click. If its more serious like one of you cheated or something, seek professional help to work through it. If this relationship is worth fighting for, fight, don't roll over. If after so many weeks/months of trying there is little change, you might have to go your separate ways. It might hurt, but in the long run its the right thing.
Now then, magickally speaking you should tell him because its kind of wrong casting over someone without them knowing. Especially if the relationship problems are trust based. Try finding spells the two of you can do. Don't know how well they work but I've seen spells to increase love/Passion, which are drinks you make together, say a chant, and drink together. If you fear something harming your boyfriend make him a protection charm. If you don't want him leaving you, there are binding spells, but those are a bad idea. Say you do break up but can't undo the binding spell, the two of you will keep bumping into each other where ever you may be. Any relationship spell should be a joint decision.