Fear aggression in maltese?

I have a 2 year old un neutered maltese, about 7 pounds. My boyfriend does not believe or allow in neutering animals, so I have to respect his wishes, since my dog was a gift from him. He has a aggression problem. Whenever my two year old sister goes near him, he snaps at her and attempts to bite multiple times. Many times she bled. He is dangerous towards her and they are not allowed to play together unsupervised, she shouldn't be near him at all actually. I'm not home often sometimes, and he stays in a exercise pen. He pees/poops in our backyard(it's pretty big for him to run around). He would only growl a little at me when he doesn't like something I'm doing to him but now he seems to be doing it more and more often. When I cut his nails, he really tries to hurt me. He growls and snaps at me. Or sometimes when I try to grab him unexpectedly, or touch him very gently. I do not want him to bite me at all, no matter what I'm doing. I don't know what to do, I love him but I can't go on like this. I have a couple bite marks and scratches on both my hands. He is becoming a big problem and I'm real concerned. I'm wondering if there's any way I can fix his aggression problem? Do you think this is common amongst un neutered male dogs? Or does he just have a problem because I am not around too often? Will walking him on a daily basis help? Will a muzzle help.. or is it just something to stop him temporarily? Would neutering be the only solution? Someone please help thanks :)