I am also a single mother (38) with a 3 year old son. I am so sorry to hear of your loss. I can only imagine what you've been through. My father actually had his first heart attack at 36, too.
I've been taking my son and going myself to therapy for a good while now, partly for court purposes. My grandmother died last year and I asked both of our therapists how to handle it. They said absolutely do not tell him that she got sick or went to sleep. They said to tell him "She went to live with God". If you tell him that someone who has died got sick or went to sleep, you cause a lot of anxiety when someone they know (or even themselves) gets sick or goes to sleep. So far for my son, that has sufficed. I'll have to ask next time what to say next if he starts asking more. He used to ask why and I would say "Because God wanted her to live there with him." I guess I would expect your son's next question to need some variation of "God hasn't asked me to live there yet." or "God needs me to stay down here with you right now."
On the dating, I left my son's father just over 2 years ago. I still haven't dated, but am planning to start. Yes, more for my son's needs than my own. It is uncomfortable, but I also miss having a man around--so it's not entirely for him.
If you're at all inclined, I'd love to get together with you sometime. It would be nice to have another single parent friend close to my age. It seems it would be particularly convenient given we both have 3 year old sons. If you're interested, please send me a private message. I'm thinking we could either meet at a local park and let the kids play or meet at a coffee shop and chat without them if you'd rather wait and meet without bringing your child out to meet a stranger. I relocated here to get away from my son's father and have a need to make new friends down here.
Hope this helps--and honestly I do hope to meet you offline.
S.