I would recommend doing 4 things:
1. Take some serious time to work on self-care, self-worth challenges, etc...especially if you're observing yourself feeling like "this guy" is the "only" guy for you, or like you'll never feel the way you feel about him for anyone else. I would say it's also important to be introspective about your attraction to men who are already involved with others, as that can often be an indication of unhealthy core beliefs that were built up from childhood.
2. Find a calm moment to tell him that you have some romantic feelings for him, and that you just wanted him to know that you're available to chat about them if he's ever single again, or if he's potentially in a polyamorous or open relationship.
3. Remember that sometimes the universe introduces us to people with a few traits that we find very attraction and appealing, so that we can include those traits/qualities in our thoughts about compatible partners who have yet to manifest in our reality yet. I would suggest that if you are finding lots of "taken" men with traits you find attractive, that the universe is saying "these guys have some traits you'd like, but many that you wouldn't like. So enjoy the confirmation of things you like while we manifest you someone with more compatible traits that incompatible ones.
4. Put yourself out into the world more; do things you love doing and are passionate about, where you can run into more men that you are more wholly compatible.