The simple answer is to get him some friends, although judging by the current stocking level of your tank this wouldnt be too much fun unless you want to get some more filtration...
And, depending on the genus of UD cat you have, they can get quite big.
We have a very shy USD cat also (he'll only get to 3 inches or so), but he's doing better since we added some live plants... I see him much more often now, but then again we've also filled in alot of the empty space inside the fake mangrove with rocks. He still has plenty of hiding places (clay pots on their sides and plants, so don't flame me!), but also seems more comfortable in the tank now...