You are so young! (Well, love doesn't choose age, does it? LOL!). For sure, something is bothering him. It could be the thought of settling down, and raising a family, him thinking of how to sustain and provide for you, specially now with our dwindling economic state - this could be bothersome.
Another thought comes to mind. "Is he really losing his freedom?"...Or could be that famous "I need SPACE" issue. There are a lot of things that happen in men's heads that are mind boggling for us, women. Your fiance is in his cave. To show him support and understanding, don't follow him there. This is the time when he wants to be left alone - to ponder.
Let me tell you how his "cave" looks like. Inside his cave, their is comfort. He finds comfort in the dark, hard, cold and stony surface. In that cave there is a dragon who stands guard - that's the reason why you shouldn't follow him inside - you're not allowed there. If you insist, you might be burned by the dragon's fire. What could you do? Wait outside the cave, and let it be obvious to him that you are not intruding into his silence.
Literally speaking, don't be too clingy. If he doesn't like physical contact right now, so be it. It doesn't necessarily mean his not attracted to you anymore. He is, but he is being stressed by work or just by anything. Let him be. Men are like rubber bands (John Gray's analysis), they have this urge to pull back. Don't interfere to this cycle. The farther a rubber band is stretched, the quicker it will spring back.
You have to have a life of your own even when you are with him, living with him. During this time, focus on your interests, add something new to your personal routine. Workout, join some women's clubs, etc. Later he will wonder and he'll feel left out, he'll be curious about you and he'll be back as your good'ol BF.
Don't avoid him though. Be as sweet as you can, but give him enough space. The rule of balance in everything is the way to go.