Is it an actual spot rather than loss of pigmentation? if so maybe worthwhile getting it checked out. It could be something he picked up if sniffing around- especially in things like soil or it could be a touch of sunburn. Hard to say without seeing a picture ( though even then would just be a guess as no expert)
If it is just the colouring gone- he could have what is known as a "Dudley" nose, which is nothing to worry about, just natural loss of pigmentation, although you may have to put some suncream on his white bits to prevent sunburn.
But if worried a quick call or trip to vets to put your mind at ease.