While I hope the humping isn't often, I consider it normal. I have very little experience with female dogs, but I believe even they will hump another dog. I have no experience with pit bull dogs but understand they have an aggressive nature and thus would be more inclined to push for a dominance order - a pack thing really, not specif to Pits.
My experience with a neutered male labradoodle oldes and female goldendoodle is the now smaller labradoodle works hard at maintaining a dominate role even though the golden is much larger - also much sweeter/gentler. I think there is little or no humping in this case, but he labradoodle did hump a neutered larger golden retriever in the past. The Golden R. has passed away and was both older and bigger, yet the labradoodle what the most inclined to hump. Here I think the lines of the black labradoodle are from more aggressive dogs, has more of a need to dominate.
I have heard that a one year-old unneutered male literally reeks of testosterone, to another dog. A one year-old is still an adolescent, with all of the raging hormones of adolescence. The older male is just reminding the younger whippersnapper of his place. It's as if the old man doesn't want the teenager to get out of line at home. As long as the younger dog tolerates it, and it doesn't start a fight, I wouldn't worry about it. For dogs to lick and sniff each other is normal, also -- even on the privates. There is a dominance ritual associated with that, too. The subordinate dog is expected to allow himself to be licked or sniffed first. Then the dominant dog is supposed to let the subordinate dog lick or sniff him. That is the canine etiquette for the situation. Again, as long as they seem to be getting along, there is no problem. Dogs lick their own privates, as you are undoubtedly aware. It's nothing gross or even sexual to them. It's just a grooming thing. If the younger dog is letting the older one hump and lick on him without protest, it seems to me that all is well.