How you clip him will depend alot upon the show. If it's a "Hairy Horse" schooling show you're not going to worry about what's "proper". All the horses are not going to look their best and that's OK. Who's putting on the show and who the judge is will also have a bearing on how you present them. If I'm showing a draft in halter under a QH judge (say a local show) I'll present more like a QH than a draft. They don't always know how to judge them and what their looking for.
Back the to question.
I'd shoot for a happy medium. Friesians have a unspoken no clipping while most everyone else is clip happy. Can you clearly see the friesian? What's the other 1/2? Personally, I clip the head like you would everyone else. They all tend to have excessive facial hair and makes them look like a goat. Get rid of all that! Make the head clean. Do tons of blending. If he's thick through the throatlatch, give him a long enough bridlepath to remove the volume. I wouldn't touch the feathers. Clip the scraggly pig hairs that run down the back of the leg above the knee. Nothing below the knee or hocks. If he's got any white legs, start working on whitening those yesterday. As far as the mane goes, (unless this is a rated or national show) I'd be pulling the mane. Find a length that works with what's rubbed out and go from there. You've got the summer to grow it out (should be able to add 4" to it's length) to where it will be a more presentable length when it actually matters. What damage are you going to do with the extensions? Have you done extensions before and can make them look natural? Less is often more.