My 4 year old son's behavior is pitiful, no form of discipline I have used has helped to correct it, any help?

He is EXTREMELY defiant, never follows directions, talks back all the time, complains about everything (from what to eat - he's a VERY picky eater - to his socks hurting him when I ask him to put his shoes & socks on everyday) and the list goes on. He has always had behavior problems but they've definitely escalated since my husband left for training in the military 2 months ago. Everyday is a struggle with him from the moment he wakes up until I finally get him to bed. He gets plenty of time to be active & let his energy out everyday because he goes to preschool. I make sure he's fed breakfast, lunch & dinner but getting him to eat healthy is a struggle because he's so picky. His idea of breakfast right now is a pop tart and a gl of milk. When he's home for lunch I'm lucky if I can get him to eat a "meal" and when I can it's either, grilled cheese, macaroni & cheese, or a cheese quesadilla... he doesn't like anything else except on the rare occasion. For dinner sometimes he'll eat a hotdog, or some noodles, but no vegetables and anything really filling. He's a healthy weight, but otherwise he would just chose to snack all day if I let him. And sometimes being on top of him is difficult because I have a 2 year old who's autistic and a demanding 6 month old who's teething. But let's talk about his behavior, oh my gosh, he's out of control. He doesn't listen, if you tell him no, you turn your back and he does it anyway, if I ask him to do something and tend to another child or try to break free to pee you come back 5-10 minutes later it's still not done, and I'm talking simple stuff, like get dressed, clean up your toys, put your shoes & socks on, etc. And I can remind him over and over and he'll get the clothes to get dressed, or get his shoes to put them on, but then something distracts him and it's like he forgot what he was doing. He doesn't like to share with his younger brother (I know this is probably "normal") and he is always taking things from his younger brother just to upset him or stir things up when things are mellowed down in the house. He adores the baby but sometimes he's right up in his face laying on him and when I tell him that he could hurt the baby he just defies me and puts more energy into doing just what I asked him not to. I'm taking him to see a psychologist right now because of how extreme his behavior has been and there have been many, many morning when I feel like I'm just going to lose it if I don't get help because I do this all by myself with no friends or family around to offer me some support when you need that nudge to remind you to "keep on going, this too shall p, everything's going to be ok". She just says she could see that he really misses his daddy, and has given me some tips on how to reward his good behavior, etc but there's been no good behavior to reward. And when he's bad I have tried everything from him, putting him in timeout, getting nose to nose with him a screaming, taking away his favorite toys, etc and NOTHING seems to be effective, it's always back to square one the next day. One thing he's afraid of is when I pretend to call a fictitious "Officer Smith" on the phone to come have a talk with him and then he gets his act together, but only for like 5 minutes. Tonight after talking to his daddy on the phone (which he does every night) he had an episode again so I tried something new and put him to bed early (8:30) and he was complaining of his head hurting probably because he was screaming and running away from me when I told him I was going to put him down. So after putting him to bed it dawned on me that this was EARLY for him, because since his dad left things have gotten a little harder for me to do by myself and by the time I pick them up from school, get them their dinner, feed the baby, change diapers, etc it's going on 9:00 or even 9:30.. so I thought, that's it maybe I need to do whatever I can to have him in bed by 7:30... which would be difficult, but I'm sure if I planned things out better or picked them up earlier I could do it. But please offer me some help!! Thanks for reading.