hi we've not spoken before, but i can relate to ur story.
my oldest daughter has a rare genetic condition and one of the syptoms is hypotonia - really flexiable muscles. she has also had the test for the muscle enzyme which came back clear. its just part of her condition. she is 4 but you can still manipulate her body like a babies you can still get her legs behind her ears! she didn't walk at all until she was 2 and a half and is still struggling now. she had problems getting up off the floor ect, and still can't get upstairs on her own. all you can try to do is improve the muscle tone as much as you can. have you been refered to a physio? you might have to push for this. if your instinct tells you something is not right, then believe it. i knew from about 15 months something wasn't quite right but no one really listened as she was still in the ' normal' range for her age. but the all of a sudden she was out of the normal range and we were then on waiting lists. my lo has been seeing a physio since she was about 2 and it has just been slow and steady progress. we have been given lots of different exercises to do to improve her muscles. there has never been a massive improvement in a short space of time, just gradual improvement.
my lo also has problems with her speech due to her poor muscle tone, and is still in nappies as she can't control her muscles, i don't know if this is the case with ur lo but i know that hypotonia can cause many problems not just with walking which is why the sooner you can get help the better.
as for genetic testing ect we knew what our lo's condition was as she was diagnosed at 4 days old which although saved her life because she got the medication she needed, it didn't give us a clear prognosis for her future because the condition is so rare. there are thounsands of weird and wonderful genetic conditions out there that cause hypotonia but knowing the name of any condition ur lo might have will not necesserly give you any more answers or affect any treatment. try not to worry too much about what it could be. it could be a genetic condition or just the way his body is. i'm sure anything very serious would have been picked up by ur paed by now. if your paed was really concerned they would not suggested waiting 6 months.
hope this helps and good luck xx