How do I tell my 12 yr old his dad is gone?

My sons dad died suddenly of a heart attack last Weds. My life has been a living nightmare since. My son has seen me cry and keeps asking me, “mommy happy?” His dad and I are still married but we separated and took care of my son 50/50. To say my son adores his dad is an understatement. Every time his dad would pick him up he would run and scream “daddy!” He doesn’t even do that for me. His vocabulary is limited and I don’t think he would understand the concept of death or heaven at this point. I don’t know what to do. I’m beyond shattered myself and the thought of breaking my sons heart paralyzes me. I wonder if having a viewing and showing him daddy would help or what would? I know his dad wanted to be cremated, so having a burial and showing him daddy being buried is not an option. Thanks in advance for any words of kindness and advice.