I have a American staffy he is 12 wks and he is always shaking and looks tired please help!!?

I got him at 8 wks he was badly nourished nd had bad dandruff and smelt real bad he has improved a lot looks much better and healthier but he's just always tired and i think hes sick! Is it normal for my puppy to shake and cry all the time?? He is very insecure hates being by himself don't know whats wrong with him!!!