Most pet store people, I am fond of saying, would not know a fish from their elbow.
Tbh, I'd be surprised if the muppet who told to you to change your water every two weeks knows what an elbow is, let alone a fish... anyway. Once every fortnight is not enough water changes. But we'll get to that in a minute.
I am guessing he a/ has been exposed to a lot of ammonia, coming from that muppet's shop, where he was no doubt kept in a tiny little cup, and b/ having water changed once a week. And c/ - most likely he wasn't acclimated (slowly introduced to his new water a bit at a time) when first placed in the tank, or after the first big water change? He could be in a little chemical shock and suffering some ammonia exposure.
He could also be unhappy with the heavy filter flow -- are you sure there's no knob or dial to slow the flow down? My filter pump has one in the most stupidly hard to find place, took me a week to figure out it had one at all... If not, you can 'baffle' the filter (using sponge or a plastic bottle, see the stickies on baffling here on the forum..) -- I've read a few threads now where filter flow was causing stress.
Try slowing the filter, and do more water changes - at least 2 x 50% per week, try to match the new water's temp to the old as much as possible and introduce your fish back slowly, adding a little new water to his cup every few mins before dumping him back in (if you remove him.. I do, in my smaller tanks). You can get a vaccum pump to vac the gravel (these are brilliant and very handy) -- suck water into the tube to get it going, put one end in a bucket, the big end vacs the gravel, take care not to suck fish into the vac - or cup him while water changing, with a lid on the cup..) My changes with the gravel vac are so quick now, just a few minutes, and the tank stays nice and clean.