You need to negotiate with her first...

She may or may not want him involved. What if she does but more than you're willing and then upon learning that, he wants more too.
Let's assume she's okay with his proposed level of participation. You may come to find you don't want to share (her) with him, even to watch. Or he may want to change his level of participation as things get hotter and heavier.

All those things must be considered. Are you to be independent in your Domme mode? Is he allowed to call the submissive you into service if you are domming her? You need to define boundaries. What if you enjoy it and want to explore domming a boy toy?

Beyond that, go for it. If it sucks, (in the bad sense of the word,) at least you won't wonder in the future. But more likely it will be great.

But remember... as you grow, so will your other relationships... and they will never be the same. They may be better. They may be worse. But they will change. (but they'll change regardless... so go for it.)