We use a star chart for our 23 month old, to encourage him to be kind to his lil sis! If he gets a certain number of stars, he gets a treat (not sweets but things like a trip to see the animals at the pet shop which he loves). Only thing I would say is that he does not seem to grasp that he has to wait for 5 stars, and wants the treat right away, but the principle is there! I am sure that it will help with your son even if he doesn't fully understand to begin with he will.
I make such a big fuss of him if he gets 5 stars that he becomes kinder towards his sis every day where previously he was slapping her a lot (she is only 12 weeks bless her)
