H.F.
I've never had this problem but one thing that helps a child overcome a fear is to make the task they are doing fun. As for blowing his nose, tell him that you are pretending to be elephants. Show him that blowing his nose makes an elephant noise. Ask him to give it a try.
Also, it could be important to find out why it scares him. Perhaps he is afraid he will blow his brain out of his nose for example. If this is the case a trip to the library or book store where you can look at a body book and show him that this is impossible might ease his fears.
The important thing is not to make light of his fears. Although they may seem silly to us, they are very real for him. It is important to help him to overcome them without belittling the fear. If we belittle the fears of our kids, then they start hiding their fears making it harder for us to be parents and for them to have peace of mind. Just some personal observations based on my experiences as a parent and as a friend watching my friend and relations husband as he belittled the fears of his daughters.
One final note, if he is afraid because it might hurt. You might try a little of "Little Noses" Saline nose spray to help break up congestion a little and make it easier to blow out.
Anyhow, good luck with your little guy. I hope this helps.