I wanted to use the word "unwaning" (or, perhaps, un-waning), as in "for his unwaning enthusiasm". However, no decent (online) dictionary I've consulted seems to recognize it. What other word would you use? Does "unwaning" sound weird, or does it sound right to a native speaker?
With the word enthusiasm, you may want the word unflagging, or some synonym:
...for his unflagging enthusiasm.
I am not sure if it's the meaning you are looking for, but "unbridled enthusiasm" is a common English expression for unlimited amounts of excitement.
If you want something more like constant and steady excitement, "unwavering enthusiasm" might also suit.