lol, when I first got my little mantis, he wouldn't eat at all for a week and hid the whole time, it worried the hell out of me because he was so active at the fish store. Actually I don't think he started really accepting food until I added the clown goby to the tank. When the goby was added, the mantis would come out of his cave all of the time and even run out on to the open sand. Now that he has eaten the goby, he is still really brave, much more so than my 4" G. Chiragra.
Adding a fish to your tank might give you the same results, I just wouldn't get too attached to the fish, my wife was ****ed when I told her the goby went missing.
Another thing you could try (if you haven't already), is to add some Nassarius Snails to his tank, they are tiny and their shells are so thin he shouldn't have a problem opening one up. They also help keep your sand clean, which is a plus, and stay hidden while your mantis is active so it's not like he'll pick them all off in a single day. Plus they are cheap. When I add them to my tank, I'll lure them out with a Cyclopeeze pellet, if the snails didn't take the bait, my mantis would just eat the pellet himself, so it works out pretty well.
Anyway, good luck and keep me posted on your mantis!