Help explaining divorce to a 3 year old

Nicole
My husband and I have been “separated” for a few months but just started the process of a divorce today. We are still living in the same house but sleep in separate rooms and have a 3 year old son. My soon to be ex husband gets a late night phone call this evening (most likely from his side piece), walks in the room and plays with our son who is settling down for bed, then walks out the house. Now I know this is a booty call because my husband is military and works morning shift and is usually dead to the world sleep by this time UNLESS he’s horny and can’t get to sleep. We stopped having sex last week when I found out about the side piece and that they were possibly having sex at the same time. Sorry got off track... my son, who is now wound back up and ready to play, jumps up and gives me a hug. He then takes off towards his dad’s room to give him a hug. I explain to my son that “Papi isn’t here he had to leave again”. My son asks “is Papi mad” I tell him no, he asks “is Papi sad” again I reply no. My son with the saddest face bolts back to my bed and starts to cry so I ask what’s wrong and his reply tore me to pieces.... “Papi doesn’t want to hug me again”..... sorry for the long backstory but these type of convos with my son are coming up more frequently and I need to know how to deal with the questions or just some general help. Thank you for reading this far!!