to answer some questions:
if the lfs is charging you for water testing i suggest finding a new place to get your fish. it should be free. if not it is clear indication that they only want your money and will tell you anything to get you to buy their livestock (ie., the tetras are a good strain").
secondly, 28 ltr is about 7 gal. those neons would need a MUCH bigger swimming room to live happily. id ask for an exchange for something else. a few pygmy cories, two or three apple snails, some shrimp, or an african DWARF frog (one with fingers on his front feet, not webbing) would be good choices for a smaller tank if you wanted some tankmates. also some galaxy raspboras or micro raspboras. i have a 6 gal very heavily planted tank with a betta, two cories (peppered and C. sodalis), two otos and a BUNCH of snails. stats fine for months and months. bettas seem less bothered by bottom dwellers ime.
you might also try a few (no more than 4) endlers in with him, although my betta did NOT like them when i tried that and acted much like your boy is acting. became very reclusive as if they were too much commotion.
as your betta is hiding and not trying to chase the tetras all over the place, i would bet that he is a more mellow fella. tankmates should be ok for him but watch closely upon introduction. but the neons, im afraid, are probably not a good choice. id suggest looking into some bottom dwellers like pygmy cories and some apple snails( which are really fun to watch!)
all the best!