Its good you're concerned and wanting what's best for your stepson. Sorry you also experienced this with your own childrens father. I lost my dad when I was two and clearly understood he was gone at the age of 5. I also recommend childrens grief counseling because I used to feel sad on fathers day when everyone else was making cards. Its also okay for kids to see others sad rather than shielding them from it all. Here's another link:

How to talk to your kindergartner about death | BabyCenter