Please don't take this wrong but maybe he needs more of your loving attention....you say it's midnight and he's still up while you're on the computer....perhaps you could read to him, help him lay down and talk to him?? he doesn't listen because you are not being a good example....he needs your love and attention.....remember children are born with a clean slate - it's us as parents that mess them up.....you get what you put in....
You dont say how old the other brother is but I gather he is younger The 5 year old could be jealous as very often they feel left out where a new Baby comes along. He is acting out because he feels sad, try to give him more attention, could his Dad play with him by himself some guy ball games and you have the younger one then reverse it you read to him and let Dad have the younger one.Spanking does not work it reinforces negative behavior and he will think he can hit out, as he sees you do it. Back off a bit ,see it as a cry forhelp and more love and you will have a happier boy, also ask his grand parents to do the same not to say he will be taken away!!If Granpa is active can he play games with your son.