This is just my opinion, but I have a feeling that a 3.5 year old will know if you are being open and honest or not. I know that from a similar age I was very aware that adults were keeping all sorts of information from me.
If he asked about something else you would give him your perspective on it, right or wrong, why not on this?
I think you are being more concerned about the feelings of believers than about your sons well being.
I would advise strongly that you do tell him about the wide opinions on what or who god is, and that there may not be a " God", a friend of mine , XJW, said to his daughter "You can bet, because there are so many opinions out there, that there is not an answer that can be proved"
I imagine she will trust his judgement on matters in the future for his candid and educative words, he did this despite risking the ire of his JW wife and family.
As he said to me," no one , not even me, has the right to indoctrinate my kids"