How to discipline my 20 month old?

[Candice]

First of all, my son's father and I are not together. My son is with me a majority of the time, but on the occasional weekends he sees his father, he comes back and is EXTREMELY disobedient. I am very stern and consistent when it comes to teaching my son right from wrong. His father, on the other hand, could care less. Though he is not even 2 yet, my son is very intelligent for his age. But it seems like he listens to me when he feels like it, maybe 50% of the time. The other half, he'll stare at me and smile when he knows he's doing something wrong. I am constantly putting him in "time out" and trying to be patient with him, but it gets so exhausting. I am disciplining him from the second he wakes up in the morning until he falls asleep at night. My boyfriend tries to help me and give his opinion, but over all I am his mother. I'd just like some advice from other young moms. My son screams, hits EVERYTHING (me, walls, the floor, etc.) when he doesn't get his way, no matter how sincere I am about it. I understand this is part of him growing up, I just need new ways to handle it! Thanks :]