How can you start the communication flowing between two partners when one partner does not talk about his feelings for anything? Should I take his silence as he’s okay with everything?

NO. This goes for poly and mono relationships, for one night stands, for friendships, for all humans: Never make assumptions about the other person’s needs. And never ever take silence as consent. 

If your partner refuses to talk about his feelings, that’s a problem that needs to be addressed, not ignored or worked around. Be clear with him that this is a need of yours and ask him specific questions about his feelings and needs in certain situations or regarding specific issues. If he refuses, ask for his reasons. How does he feel when asked to share his feelings? Is it that he’s afraid, or that he genuinely hasn’t done the work to figure out how to articulate his feelings? Don’t push, argue, or accuse. It’s your job to make him feel safe and understood.

Consider finding a couples therapist, making sure beforehand that if you’re LGBTQ that they’re LGBTQ friendly, and if you’re poly or exploring polyamory, that they work with that too.