First off — if he couldn’t be bothered to tell you about his other relationship, he was not truly practicing polyamory.
The single most important factor in poly relationships is communication. Without it, monogamous relationships crumble. Poly relationships are impossible if the people in them do not communicate.
It can and will work — but you touched on the other key. Maturity. You have be able to share and, again, communicate frequently. But a good poly relationship is not so different from a friendship — and most people have more than one of those.