My 5 yr. old is very mean to his 2 yr. old brother every chance he gets he hits him what should i do?

my oldest son paul has special needs. he's not quite like other kids he cant talk and is developmentally delayed. doctor are not sure what he what is wrong. and im not sure exactly what he understands because he cannot tell me. anyway my youngest nathaniel will be walking around minding his own business and his brother will reach out and smack him, kick/trip him, clothes line him with his arm, or something. i don't know what to do. they are kind of mean to each other (taking a toys from the other) i usually try to keep them off each other but paul just goes out of his way to do something to his brother. i holler at them a little to much maybe but im at my wits end. it is probably jealousy issues but what do i do about it?