This will be a bit hard to explain, but I'll give it a shot.
I am looking for a good word to describe: A person hiding his intense positive enthusiasm and excitement for something, as to not seem overly excited and hence being perceived as emotional or irrational. This person is afraid that his excitement might generate some sort of mistrust with the cautious person who will decide if he will be able to experience what he is exited about or not.
"He is a true pragmatist; one who takes on new challenges with great, yet [word here] enthusiasm."
I have been thinking about using concealed, reserved, humbled, restrained and modest, but I feel like those words come with a sort of negative undertone, as if the person is harbouring some kind of hidden agenda, which is not what I want to say. Any suggestions?