Re: discerning right vs. left, he's still young - it would not be reasonable to think that he should know this at this point.
I know that there is a lot of variability to be expected. He missed the expected milestones for number of words at 15 months and 18 months-I think he was in the very small minority- this had me very worried. We took him to the speech expert and they recommended speech therapy. We did this, and pretty soon he was talking up a storm. I don't know if it was the therapy that did it or if it was just part of the variability-(Now he talks a lot (a lot) and is very very active-to the point that I have wondered if he might be "hyperactive". (I know that there is lots of variability is to be expected). But...
He is now 4 years and 7 months. So, the "most children" has me a hair worried. I would of course prefer that he doesn't miss these neurological milestones. I don't want to wait if there is something we could do now that would help him. I would highly prefer if he didn't start kindergarden way behind the curve-kids can be pretty unkind to other kids.
Caution should prevail whenever we indicate an age at which children lean something. There is a wide spectrum of normal. Having said that, most children by the age of five, and certainly six, know left from right.