Welcome to the forum! You will find a lot of support here! Wow! You have been through a lot! I hope you find comfort here.
The majority of people in this culture do not understand polyamory. I guess it's best to keep it confidential for the most part. If you need validation for your relationships, get that validation from people who do accept it- like people here on this forum.
Another thing that is important to understand- and I'm sure you have given this a lot of thought. The grief process is quite difficult and when a person is in the beginning stages of grief, they really aren't available for a healthy relationship. He really needs time to heal. I understand how compelling the attraction must have been, but be careful that you are not "rescuing him" so that he doesn't have to do the difficult work which is ahead of him.
The key to life is in being fully engaged and peacefully detached simultaneously and authentically in each moment.