D.O.
My daughter is 19 months old and she hits too!! She started a little before 18 months. She hits EVERYTHING... people, the floor, doors... whatever is closest to her when she gets mad!! I think it's just that they don't know how to get across what they want or deal with their feelings yet...
Unfortunately they are still kinda little for a real time out and understand... but I try this sometimes...
When my daughter hits I tell her a firm NO and put her hand to her side. Then I put her down and walk away from her, or if she's down I will just walk away... they really don't like having you withdraw your attention that way. No matter how she acts after I walk away I ignore her unless she is doing something that can hurt herself, etc....
I have noticed that she doesn't hit me now like she used to but my MIL who doesn't do this gets the heck whacked out of her sometimes!! So I am gonna have her start doing it to... she will still whack inanimate things but is learning that hurts her hand... so I think she will hopefully find a new way to deal with frustrations soon!!
When the other kids are around if he is doing this I would do the same and put him down but isolate him somehow from everyone else...
Good luck... I hope things get better soon....