Welcome to our forum.
It's a bit of a puzzle how to reassure your partner when he gets feeling insecure. Perhaps the uber-direct approach? i.e., having a sit-down with him and asking him straight-up what you can do to reassure him.
The one other thing that came to my mind is the book, The Five Love Languages: how to express heartfelt commitment to your mate ... by Gary Chapman. It's not necessarily gay-friendly or poly-friendly per se, as it comes from a conservative viewpoint. But it's a pretty mild-mannered book, and it makes a good point: What "feels like love" to one person, isn't always what "feels like love" to another person. People speak in different "love languages."
The love languages Gary Chapman mentions are:
- Words of Affirmation.
- Quality Time.
- Receiving Gifts.
- Acts of Service.
- Physical Touch.
Perhaps there are other languages not mentioned in that list, who knows. The point is, maybe trying to pin down which love language your partner "speaks" would help. You might be speaking to him in the "Receiving Gifts" and "Words of Affirmation" languages, whereas he might need more "Quality Time" or "Physical Touch." Might be a helpful idea for you, might not ... but I thought I'd mention it just in case.
Keep working on communicating with each other, both the talking and the listening part. I think things will get easier for you eventually.
Glad to have you aboard.