Hello,
I am happy to see that you are objective that it is his BEHAVIOUR and not his personhood that is the problem. A MISBEHAVED CHILD IS A DISCOURAGED CHILD. Toddler children because of their lack of verbal skills tend to use physical aggression to try and get their point across. This little boy sounds like he needs the attention and so he knows that by hitting he gets some attention. Have you tried giving him some positive attention during the day? For example, if he is playing nicely for a bit you may want to comment on what he is doing by asking open-ended questions and being genuinely interested in him. Try and see if you can provide more-one on one time with him. If he sits on another child say "G you need to lie for a bit then lie down on this nice big pillow I have for you." Providing opportunities for using his hands in a positive manner such as having him play with playdough or other sensory materials to take out his aggression on. I like playdough and goop as it allows children the need to use their fingers and hands by pounding the playdough and squeezing the goop. Canteach is a great website you can use to make these sensory activities. I hope this helps. It is gonna be okay. I think getting together with mom to think of ways to help him through this agression will help. As her what he likes to do at home using his hands (aside from slapping her )