There are a couple of fun games you could play with him to try to get him practicing to blow. Get a feather and put it on the table. He has to blow the feather off the end of the table using his nose to blow it. If he is really reluctant to try maybe try offering a reward or treat. I'm not usually a fan of rewards but there are times when it does work.
Another game you could try is making some playdough birthday cakes only use real candles and get him to practice blowing them out with his nose.
Try to make it fun without to much pressure but explain that you need to practice blowing your nose to get good at it just like drawing or riding a bike etc.
Hope you find something that works for you.