Social stories! They've been a lifesaver for us for all kinds of behavioral and/or habit issues. Here is a link with a ton of resources with websites for social story making and help:
http://www.theoatc.org/resources/documents/Reso...
Also, depending on how much he understands, you can also tell him that nose-picking is a "private" thing - that if he feels he must do so, to go to the bathroom to do it!
Good luck, Anne

I had heard that nose picking might be a "nervous" tactile response. If you could put something in his pocket that he can constantly "feel," like a little balloon filled with sand or a cotton ball, etc. and keep him occupied with something else besides his nose, this habit might go by the wayside. I tried this with my son (who was always playing with his genitals or picking his nose) and it did help.

This is a tough one, for my son it was picking at his face or eyebrows. I'm all about reinforcing appropriate behavior, so maybe set up a token chart and for every hour or so you DONT see him picking- give him a token. Set it up where he earns something special for however many tokens. The other thing is that he needs a replacement behavior, maybe something to do with his hands. Maybe a fidget toy?

social stories and maybe a humidifier