At 20 months I am pretty confident it is out of frustration. Boys especially handle things physically and impulsively. They don't have the words yet and at this age it is very narrow scope that the world revovles around what they want! :)
The best thing that worked for my son at that age was to catch his hand, firmly look him in the face eye to eye and say very strongly "NOT NICE!!!!" and lay his hand down to his side. My son was devestated if I got upset with him so it only took a little while before it kicked in his brain. He used to grab his sisters hair and rip it out when he was this age and I scooped him up and put him away from everyone and told him firmly "NOT NICE"...
it won't be a magic overnight change, but time outs and being seperated from the situation is really the best at this age. To be removed from whom he hit is a signal that that he is not going to get what he wanted or play if he does it, it may take 20 times.
If you can, when you see him ready to do it, catch him in the act and gently but firmly grab his arm or hand and stop him!
My son's vocabulary took off after he turned two, with a older sister your son's probably will too, until you can teach them how to better handle the situation with logic and words, just stay consistent with removing him, being firm and setting him away from everyone! :) It is a phase.
My daughter used to bite, my son used to throw toys and pull hair! :)