He is disturbed that the Rebbe did not personally sign his name when the Rebbe responded.

The Rebbe assures him that he personally reads and dictates the answers and explains why sometimes he signs and sometimes he doesn't. Additionally, the other letter, in which he is included among the recipients, did have the Rebbe's signature. https://www.torahrecordings.com/rebbe/igroskodesh/011/010/3679