The best I can offer you is to give it time. You may be fine throughout the day, but that's probably your body trying to defend your emotions. I take it your someone that rarely shows your emotions to others? Do you view emotions such as sadness or crying as a weakness? It's just a thought... Either way, there's no reason that this ~shouldn't~ affect you... so while I can't make the pain go away, I just hope you realize that how you feel is normal. And that at night, when yuo're by yourself, you just want to let it all go? Then let it go... It's hard to face the reality of the situation, but with time, you'll find that you're able to start coping with it... and instead of sadness when you think about him, you'll remember the good times you spent with him. That's what I believe anyway.
My condolences to your and your family.