L., no 20 months is not too young to start training, in fact we should train from infancy , but i know you mentioned you threatened him with a time, out, dont threaten, do it, if you say something follow through with it, it will help in consistancy, make your yes mean yes and your no , no, this helped me and my 3 boys, also a time out for a young guy like him should only be a little bit, he does not need 10 minutes, just simply remove him and sit him somewhere helps a big deal, dont make it a game, when he gets up , go ahead and let him, and what you are saying is good too, they say give them 1 minute for every year they are, so he gets a little less than a minute, and stay there and watch him, when time is up , let him go, my kids used to argue when i would send them to their rooms, i finally said, if you dont go to your room, how can i let you out, they would go and ask can i come out and i said yes, it worked, cause all i needed was a brief breather, so time out or discipline does not have to be harsh, or long, at his age i think removing him from area is good, and no threats, if you say you are gonna do it , do it, if he hits the dog once, i would do what you do, if he needs longer time out , stand there with him and or sit with him and hold him, then say ok you can get up if you dont hit the doggie, it seems like you got it handled, keep being the good mom you are, and dont allow hitting, enjoy life and have a good day , D. s