First off, congrats! It's good you're moving in together before the wedding to see how you co-habitate. Presumably he's stayed over there before, so you've dealt with stuff like sharing a bathroom, bed and maybe a dresser drawer or too. Now comes ALL HIS STUFF and that can seem daunting. Plus, he's moving into your place, so he's out of his element. Here are some tips for a smooth transition:
1. Give him his own space. Whether it's a bookshelf, dresser, office, closet, etc, give him an area where he can store his stuff. Sure you'll be melding your stuff together, but he still needs a little of his own space even if it's a rack for his Blu-rays.
2. Let him rearrange or decorate something. If he wants to turn part of the living room into a home theater, let him. Give him a project to do. He won't turn the whole house into a man/nerd cave. Maybe just a room or so.
3. Don't sweat the small stuff. You're going to be living together presumably for a long, long time. There is time to redo the kitchen and buy a new shelf and all the things that couples who live together need to do. Enjoy your (literal) honeymoon phase of living together as a newly engaged couple. If his dining room table doesn't arrive on time, eat pizza on the floor together like people in movies who move into a new place. Enjoy this new stage of your life together.
4. Go shopping together. He's moving into a place filled with stuff you've accumulated. Now it's the time to get stuff together as a couple. Kitchenware, paintings, couches, bookshelves...whatever you need, now is the time to get it together.
5. Don't let the inevitable IKEA trip tear you apart. Remember it's just a store for cheap Swedish furniture. Your relationship is stronger than that.