H.B.
My grandma had a miscarriage when my uncle was around this age. He knew about it and she used the seed and garden explanation also. She just told him that she wasn't sick, it was just a bad seed that didn't grow right. Just recently I've been slowly preparing my 5 year old that I might have a C-section. I've been watching the baby shows on tv and he was wondering why some mommies are being cut. I went with the straight answer, giving little information at a time until he's satisfied. I explained that it was normal and we trust the doctors, they go to school and live their lives doing this. I trust them to cut me open before I trust daddy to cut me open. (Plus the doctors tools are much cleaner than daddy's) It helped him to know that doctors are real people to...they don't fix roofs like daddy, but they help mommy have a baby!
Anyways, let him ask the question and answer it the best you can. Don't go any farther than what he's asking. Hope some of these suggestions help. Hopefully just talking about why he's scared will help him start to feel better.