If you cannot find your 1850 ancestor in the 1840 census–and you are certain he’s heading his own household–consider searching for his 1850 neighbors in 1840. Then look at their neighbors in 1840. There is a chance your ancestor is near at least one of his 1850 neighbors in 1840. And there’s also a chance that your ancestor is not the head of their own household in 1840, but is hiding in one of those tick marks for age categories.
And it’s also possible he was overlooked entirely.