Mark, my son was 8 when his Daddy passed away. He would sit and throw temper tantrums at anything I would try to discipline him for. A simple, "No, you can't have candy now" and he would kick the stairs and scream like he was in pain. I wanted to hug him as hard as I could to take away the pain, and I did, for a while. However, after a time, I had to make the hard decision to discipline him. I remember sitting in the corner crying and shaking, waiting for his tears to stop, for him to calm down. He couldn't see me, so he didn't know that it had an effect on me. I just sat close, where I could hear him and when he calmed down, that's when I would go to sit with him.
I'm trying to say that it's okay to acknowledge that he wants Mommy, but don't allow him to use that button on you. You can say, "I miss her too", but right now I need you to focus on putting your stuff away. Or whatever task you are doing.
My son is now 14, and the single parent thing is still HARD!! I don't know how to do calculus, I can't help him in some of his school work. His Daddy would've been great at it. I just have to use what resources I can, (tutors, etc.) to help me instead. Yes, the friends and family help goes away quick.
My son is also afraid every time he gets sick, stomach pains, etc. He thinks he may have cancer too, he's afraid to die. It's been almost 6 years for us, and his fears are still there. I catch him Googling medical stuff, he wouldn't even use the Acclear acne medicine I got for him cause it contained Dimethicone...it can cause cancer. I did try to explain to him that he isn't exposed to it in a massive amount of levels, so I'm sure he's fine. He still threw the brand new tube away. :-\
No, it's not easy. You are not a "Single Parent", you are an "ONLY parent". Yes, there is a difference. You are the only parent he has, you'll do great and you both will and can "grow" together.