My 2 yr old son is acting out terribly, is it me? what do i do?

My son will be 3 in April and he will not listen to ANYTHING I ask him to do. He is acting out at preschool and I even got a note from the teacher yesterday saying that he was slapping and kicking his friends, tearing books, not listening, yelling at teachers, and running out of the room. To me this seems like he is acting out and its not just misbehavior. I don't want to say that he is bad ALL the time because he can be the sweetest little boy when he wants to. He just does NOT listen! "Take your shoes to your room please" "NO! NO! NO!" his response is always no! I try to discipline him the best I can. If he says no and throws a fit then I put him in his room for time out and he cannot come out until he is ready to behave and stop crying. Right when we walk in the house from work/school he immediately goes to the pantry or the fridge... "I want cookie.. i want i want i want." Its driving me insane! my patience is running very thin and i just dont know what to do! The other morning i was getting ready for work and he had just woken up and was holding himself so I told him to go potty (which by the way he is perfectly trained) and he yelled NO! so i ignored it since it was 6:30am and went to finish getting ready. he walks in the bathroom and asks for candy (valentine candy) i said "kobe we dont eat candy in the morning, i will get you some yogurt" he then slams his toy and blanket down on my foot and broke the head off of his 30 dollar toy. i take him in the living room and sit him on the sofa when i notice a hole in my wall! which im uming came from him throwing something. im completely lost. i feel like im trying my hardest to guide him in the right direction. i work a full time job and im a single mom, i dont get to spend a whole lot of time with him and i wonder if that has anything to do with him acting out. ALSO, a large factor... I have been dating someone for the past year and about 2 months ago he moved in with us. He is very good with kids but he disiplines with "the belt" and time out. I have recently let him somewhat take charge when my son is acting out since my son just does not listen to me and he also does not have a father figure around. My son will listen to him but I feel like he comes back to take it out on me because my discipling is more "soft." if he tells me "no" my boyfriend will come behind me and will talk to him about listening to me and not telling me no. does he resent my boyfriend? is he just adjusting to him being around? does he feel like im paying more attention to my boyfriend than him? everyone tells me i need to whoop him more but i feel like i whoop his almost everyday and nothing is accomplished! im just tired! i feel like i yell entirely way too much and i know my boyfriend is tired of hearing it because i am! im getting worn down! one more quick little peek... the other day we were at the mall with my ex mother in law (my sons grandmother, which we are very close) and we were buying my son shoes. he ran out of the shoe store into the mall, i couldnt find him for a very long minute and then i spot him running out of the candy store with candy in his hands! usually i will wait to pop him but i did it right there on the spot! later on, while im in another store his grandmother goes back to the candy store and buys him the candy!! completely unacceptable! she spoils him rotten and then im the bad guy. please help!!!!