First of all, do not try to cover up your body with baggy clothes. You have assets and you should not hide them underneath a bunch of fabric. The excess fabric will only make you look bigger, so you may as well embrace the fact that you're how you are, even if this is only a temporary condition. Your formal suit or tuxedo should be fitted to your current body—do not buy it in a smaller size expecting to lose weight. The occasion for which you are preparing is important enough that you want to look really nice. You will be stressed out enough without worrying how you are going to squeeze into that tiny cummerbund.
Now, just a word to the wise: If you decide on a color scheme that is unusual, remember that light colors tend to make you look bigger. In fact, darker colors will be more forgiving as they do not create obvious shadows and wrinkles where you may have some bulges. Black is always a chic color to wear and you will find that it's always welcomed.
You can also experiment with different textures of fabric, but it's important to remember that shiny materials can also be an enemy, accentuating the aforementioned bulges you may discover you possess. Horizontal stripes are also a no-no, as are breakups of color (such as a jacket being a different color than the pants), as these all cut you into thick, wide pieces.
Another important element of finding the perfect formal outfit is time. You need to give yourself plenty of time to find the right clothing if you can possibly do so. Your size can present some unusual requirements for rental places or suit shops and you'll want to leave enough time to either reserve the outfit or get it tailored to your own big, handsome body. Again, the last thing you need to be worrying about during the time leading up to your event is what you'll be wearing. You will have much better things to think about when the event nears than how on earth you are going to get your clothing.
Also, shop where you are comfortable! Don't go somewhere where you feel the staff is all looking down on you, be it for the clothing you're shopping in or whatever. You will not make clothing choices that are true to who you are and how you really want to look if you are somewhere you feel ill at ease shopping.
Now go out and get something that looks good on you!