My nephew who is now five had the exact problem your son does between the ages of two and well...somewhat to this day. He hits, throws toys, etc etc. he began hitting before his sister was born and now hits her, the time-outs don't work, explaining doesn't work, spankings do nothing...He has hit my daughter and was not affected by her crying at all. His parents are moderately AP and have moved into the gentle discipline realm after I inadvertanetly brought it up and they subseqently researched it.
Now I doubt your son has a problem but does he have any other odd behaviour? Most of the time this is perfectly normal but if the behaviour is extended, seemingly random, unceasing for up to 12 months, and inalterable it may be wise to examine any other oddness. My nephew is suspected to have a variation of Asperger's. The hitting issue was the reason my brother in law really reached out to discover what exactly he could do to help his son. Now I am not saying your son has anything like this , he is probably just going through a stage that is very typical of small children. (today my two year old daughter told my husband she liked hitting him and she has never been hit, spanked etc) I am just saying if there is any other odd behaviour it may be worth reading up on some other things. I really don't want to freak you out I just saw what my nephew dealt with and discovering this has really helped his family.
Have you tried showing your son the consequence of hitting? For example my daughter is motivated to not hit because I do not hold her while she is hitting and if she hits while we are in a store, or while I have to hold her like in a parking lot etc then we leave or I turn her facing out (she hates this) also if she hits just to get us to leave the store then we wait until she has calmed down and we attempt the store again. This seems to be working, we also discuss that she cannot use hitting as a way to get what she wants, hitting hurts mommy and mommy cannot hold her if she is hurting mommy etc. We work on demonstrating appropriate behaviour, being gentle, hugs, talking about what she is doing, etc. I do not ignore her or spank her but try to show her that hitting alienates her from things she wants to do.
We also give her items she can hit and throw then demonstrate how to throw them etc and give her ample opportunity to do so. I try to make up a lot of games about throwing, basketball, etc. When she hits her cousin (happened once) then she has to sit on the couch and not play with her cousin until she can play without hitting, now this works because my dauther is very social and not being able to interact with people is the worse thing that could happen in her life, she also understands what she is doing and is very verbal so she can express her emotions with words very well. I imagine that tt would be much more difficult with a child who was more physical and less verbal with their emotional reactions.
sorry so long, i rambled a bit.