My DD 2.8 asks all the time.. My mum died in December so I have explained that nana is in heaven. We get the same questions over and over.. Where is nana, etc. And usually followed by I miss my nanna, you miss your mummy don't you.. Don't worry, I'm here mummy...
It makes me cry, but I don't hide it at all. We then talk about my mum (what DD remembers) and then asks about DHs mum and dad (who she never met). then my dad (who she never met).
She has asked to go to heaven, but I just explain that we don't go there until we die and I don't want either of us to die just yet!! and laugh about that...
She doesn't really understand, as she sometimes says that we will see nana soon. I think she has dreamed about her as sometimes she says she has done something with nana that I know that didn't happen. (like nana took me to nursery)etc when DD didn't go to nursery until my mum was very ill etc.
Nikki, I don't think it is bad for your DS to see you upset sometimes, and it may well help you in the longterm talk about your mum etc...
I guess I am lucky as I do believe in heaven etc, so just explain to DD what I believe etc...