If you want to know how it will look just comb it down then look at his neck from the other side. If you roach his mane but want to leave it about an inch long, it can take real talent with the clippers to not get a chopped look. As it grows out you'll be dealing with a long brushcut. You might want to consider trimming his mane to about 4"-6". It will be fairly easy to groom and there's enough for braiding if you show, or roach his neck clean. It loses the freshly shaved look within days.
I, personally, like to shave it all the way down about a month before shows so it's about an inch by that time and then just keep it trimmed at whatever length you prefer.
As for shaping it, you'll be able to see where he could use some shaping after it's been roached and then shape it as it grows in. Posted via Mobile Device
I wouldn't roach it. He actually has a mane for a reason, and I don't think you should take it all off. I do keep my mare's mane shorter during the show season, but, I still leave her some mane.
Also, you do know that even with no mane, your boy is still going to roll in the mud. So you're still going to have a muddy neck to look at, with or without a mane. Horses love mud- especially white ones. Just let him be a horse.