I've found the easiest way to have my ferrets stay on my shoulder is to wear a shirt with a hood attached to the back. Unless you have very broad shoulders where your ferret can comfortably sit, you're going to need a place for him to rest. It's not fun for your ferret when he has to do a balancing act to keep from falling all the way down there to the ground. Look at the size of you and then look how short your ferret's legs are. He's used to being pretty close to the ground. That's a long way down from your shoulders to where your ferret belongs.
I read in a ferret book that the best way to teach your ferret to stay on your shoulders is by placing a can full of crumpled paper near you and letting your ferret fall in the can. They claimed the noise of the paper would frighten your ferret so much that he would stay up there from fear of falling into that noisy can again.
Well I don't know about your ferrets, but mine love playing in paper. If I gave them a whole can full of crumpled paper I'd have trouble getting them to come out.
I taught mine to stay on my shoulder by wearing a hooded sweat shirt and offering them a treat. When they wanted to get down, I put them down. If your ferret starts moving around a lot there's usually a good reason for it, they have to relieve themselves. If you refuse to let your ferret use the litter box when he wants to, you're going to find a surprise in that hood you have on your back. I don't suggest you reach in there to find out what it is.
Just a word of warning, always expect the unexpected. If you're planning on taking your ferret outside while riding on your shoulders, always have a halter and leash on him. If he did manage to fall from your shoulders while you were crossing the street or on the sidewalk, he could run off and be hit by a car before you're able to catch him. Please don't take any chances, he's depending on you to keep him safe.