erm ok, 1st of all, that is not a 15g tank lol it looks more like a 55g so you prob safe for a couple more month. 2nd you prob only need to do a 20-25% water change 1nce per week, the water looks sweet to me. 3rd you should prob try feeding 1nce every 2 days or very small amounts every day. for his size there seems to be alot of room for him so really why were you worrying again??
RTC'S tend to hide alot when small anyway so yeah he will come out of his shell eventually, just bare with him and be consistant in what your doing and he should be fine dude

nice lil monster

Just remember to be prepared in a couple of months by getting a much larger tank for him