I found a fun site for missionary gifts. It is called
Absolute Ties, which has fun ties and socks. I know the missionaries cannot wear gaudy ties, but there were a few conservative Halloween ties that I thought might be fine for a missionary to wear.
I know this one has ghosts, but I love the words: this is my costume. I then chose the pumpkin socks as they were just cute.
I ordered these and they were sent out the next day! He had them within the week.