Forgot unit number

Help! I sent my penpal our first letter in July and received one back within a week. I sent another letter back, but I didn't realize I was supposed to add his unit number in the address, because I don't with a different penpal. It's now been over two weeks without a response and I'm worried he either hasn't received it, or they threw it out. How long will it take to get to him? Can they just throw your letter away? I hate to think he's sitting there thinking I randomly decided not to write back!