It's a hard one but I think it depends on the age and level of understanding the child has, like you said.
Kids are amazingly aware of what's going on around them though. Let's just say, for arguements sake, that something did go wrong, your son is old enough to want to know why mummmy isn't well / in hospital etc.
We told our kids from the start with the last pregnancy and when I miscarried they knew what was happening and we all dealt with it together. I think it would have been worse if they hadn't known what was going on.
This time around we told them at 8 weeks and although I was scared of having to dissapoint them again, it was great to let them tell family and friends our news.
I think you do what feels right for you at the time.