IMO 19 months is too young for the naughty step, especially in this situation, when DS1 is fearing you have rejected him.
DS1 was 2.5 when DS2 was born, and also used to hit him. This came as a complete shock to me as he had been a very non-aggressive child up to that point. I don't have all the answers, because I know how distressing this is, but looking back I see how angry he was (furious, in fact), and had no understanding of what had happened or how to express himself any other way. He may look huge to you now, but believe me, he is still a baby himself.
Try to remain calm, keep your voice low, get down to his level, look him in the face and say "do not hit". Then offer him an alternative eg say "this is how the baby likes to be touched". Give DS1 as much attention as you can, especially when feeding the baby. Try not to worry that you are not giving the baby as much attention as you would like, or you gave DS1. In my experience, they get as much out of watching you play with the older one.
Don't expect DS1 to be very interested in the baby, but give him lots of praise when he does, and is kind.
I wouldn't advice a tap on the hand, as this is giving a mixed message about hitting. I know that this is not designed to hurt, but I really don't think it works
Never leave them alone together.
Do not fall into the trap (as I did) of worrying that DS1 has turned into a monster, or will always be like this.
I think that the naughty step will bring more heat to the situation. I read a book that advised trying to sound like a bored policeman when you tell them off!
Good luck. I know how hard it is, but it will pass. Mine are now 5 and 8 and get on really well (most of the time).