The inside of each nostril is a 10 value that softly merges into the edges. The edges of his nostrils are light and get even lighter near their outsides. Use a B or 2b lead to darken the entire 10 value area of each nostril, then use the tip of a stump to gradually soften and blend that dark value out into the nostril edges.
The wrinkles on his nose between his nostrils are very soft and undulating. To create the wrinkles, tap the highlight areas with a kneaded eraser pinched into a point until they lighten. If they become too light or are shaped wrong, correct them with a 4h or 5h pencil using little hatching or tapping motions.
Blaze Detail
Use a 4h or 5h lead to draw dark, sharp details into the white blaze area's edges.
Use a stick eraser that you've cut into a wedge shape to erase white details out into the dark areas on both sides of the blaze.
Both the light and dark zigzag details are sharp and jagged looking. Use a 4h lead to reshape them if you need to.
Mane Forelock
and Ears
Notice that most of the strands of hair in his forelock have soft edges. The guide lines you drew are probably sharp or crisp looking so you may need to blend and soften them. You can do that by gently tapping them with a kneaded eraser that's been pinched into a point, or by rubbing them with the point of a stump until they blur slightly.
If the highlights of the hair strands are too light, go over them lightly with the tip of a stump until they darken to the right value.
To make the hair strands more realistic looking, use a 6h or 7h lead to thin some of them. To help keep the edges soft and varied looking, thin them by many small hatches along each edge instead of a long solid line along the entire strand.
Use a stick eraser that you've cut into a wedge shape to lift out light valued strands, and to touch up any highlights that need it. You can use a stick eraser to gently lift out fuzzy hair detail in dark areas too, like those at the bottom of the ears.
His ears are an extremely dark 9 value but they do have some highlight detail. Fill the ears in with a 2h around their edges and a 2b on their interiors. Use a stump to pull graphite over into the highlight areas then use a 4h to even out and darken the highlight areas even more. The highlight areas are very dark and soft around the edges so blend carefully and use a kneaded eraser to lift out graphite if they become too dark.
Mane, Neck, and Chest Area
His neck area, next to his left cheek, is 9 or 9 1/2 value. Use a B or 2b to darken in that part of his neck. Continue filling in those dark values up into his mane hair. There are some muscles at the bottom of his neck that you can help define by using a kneaded eraser to gently lift some graphite over their tops.
The hair strands low on his neck almost completely blend into the shadows of his neck. Notice that the shadows under his mane, low on his neck, are 9 and 10 values, but the shadows near the top of his neck mane are 7 and 8 values. That's because the sun is above and behind him and the light washes over his body from behind and over the top. You can see this pattern in all the shadows and highlights of his mane. Both shadows and highlights are lighter near the top of his neck than near his chest.
You can use a stump to help darken in highlights in a gentle way if they are too light. Use your sharpened stick eraser to bring back out highlights that need lighted again and to help create hair strands. Work back and forth with your eraser and stump until the highlight and shadow patterns seem right.
Use your stick eraser to lift out the mane strands on his neck. If the strands look too large or aren't shaped right, use a hard lead to redefine them. Finally, use your 6h or 5h to help create delicate thin hair detail all over his mane. You don't have to draw each hair, just a few will be enough to make it look like hair.
Finish
To make the portrait look more finished, make the left side line of the horse's chest smoother as I did here. You'll probably need to clean up the paper around your portrait, too. You can use you're stick eraser to do that but be careful not to erase any hair detail.
The inside of each nostril is a 10 value that softly merges into the edges. The edges of his nostrils are light and get even lighter near their outsides. Use a B or 2b lead to darken the entire 10 value area of each nostril, then use the tip of a stump to gradually soften and blend that dark value out into the nostril edges.
Use a 4h or 5h lead to draw dark, sharp details into the white blaze area's edges. 
Notice that most of the strands of hair in his forelock have soft edges. The guide lines you drew are probably sharp or crisp looking so you may need to blend and soften them. You can do that by gently tapping them with a kneaded eraser that's been pinched into a point, or by rubbing them with the point of a stump until they blur slightly.
His neck area, next to his left cheek, is 9 or 9 1/2 value. Use a B or 2b to darken in that part of his neck. Continue filling in those dark values up into his mane hair. There are some muscles at the bottom of his neck that you can help define by using a kneaded eraser to gently lift some graphite over their tops.
To make the portrait look more finished, make the left side line of the horse's chest smoother as I did here. You'll probably need to clean up the paper around your portrait, too. You can use you're stick eraser to do that but be careful not to erase any hair detail. 
