Just for clarification, are you planning to date him until you find Mr. Right, and then drop him like a ton of bricks?
If so, make sure he's aware of that. Also be aware that finding Mr. Right might be more difficult if you're in any kind of a relationship, if Mr. right is not polyamorous and might see you as cheating etc.
However, if you're planning to explore the polyamorous lifestyle for yourself, then this sounds like a great introduction to it. You've met someone who has set clear and reasonable boundaries for his life and what he has to offer you. You know that he's engaged and has another girlfriend, so you basically know where you stand and where this is not going (i.e. marriage).
I think it may be easier to experience poly the first time as a non-primary partner of an experienced poly person, so that you don't feel like you're losing something you already had, but rather gaining something you never thought was possible. Not that mono-to-poly is a loss, either, but it can seem that way any time big changes happen...
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker