Why is my soldier not receiving my letters?

My boyfriend is currently in basic training at fort Knox Kentucky. He sent me a letter with his address a couple weeks ago since then I have sent him about 6 or 7 letters and his mom an sister have sent him about 3 or 4. I got a letter from him today asking why no one has written him. I wouldn't be so worried but he is the only one not receiving his letters. And we have the address perfect. Is there anything we can do? Please help!!