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... after my knight boosted my endurance from 15 to 19?
He used to be able to get 14 hit points max, now the max I got was 16.

So that's it? Only 1 point more for each "threshold" attained in endurance for my knight?

In MM6, each threshold increment was then multiplied by a constant, so warriors benefitted more from endurance boosts than mages did.
In MM1-MM5 each class gets the same bonus HP for high con.
kmonster is correct. However, the character's class still affects the base HP per level, so your knight will always be getting more HP per level than your sorcerer for example. It's just that the endurance stat adds a modifier to this that is the same for all classes.