How to explain to my 6 year old that his dad is in prison

Everything is finally hitting me and all I can do is cry. My husband is in prison and got 15 years. My 6 year old doesn't understand why Daddy is away and why mommy is sad.

How this happened is my husband had an affair with another man at his college. From what I understand that man didn't know he has a wife and kid because he told nobody there about us. They were just having sex. The man said he wanted more of a relationship and he was actually in love and for some reason that made my husband snap and physically assault him and he passed away on the way to the hospital. That's the short version of everything. It's all really complicated. It ended with my husband taking a deal to plead guilty to a manslaughter charge instead of going to trial for a 2nd degree murder charge.

My husband and I haven't spoken in 2 months even though he's tried to call me and our son keeps asking where is daddy and idk what to say. He got picked on at school by a 4th grader saying his daddy abandoned him which has made him really sad and he's asking why I'm sad and part of me is thinking about lying. Maybe saying his daddy is away doing something great. To preserve his innocence. I just don't want his father's actions to effect him