My heart breaks for you, and although our situation is a little different, I may be able to offer support as an adoptive mom. We adopted my son last year unexpectedly when his grandmother who was caring for him, along with occasional support from his birth mother, passed away unexpectedly. We have had him for over a year now, and while I am very open to keeping in touch with his biological family, especially since his birth mother is part of my husbands family, sometimes it's hard. My son's paternal family are a messed up group of individuals, and I did try to maintain contact with them, but due to drug use, and some other issues, I have made it very limited, to protect my baby.
What I can tell you is.. there is a fear as an adopted parent that someone will come and take your baby away. That it is hard sometimes to be reminded constantly that you didn't give birth to your child and there are others in his life that know that. It's a very hard thing to admit to. It's selfish and unfair to others who love him, but none the less, it's been true for me. I talk to my son's birth mother about once a week, and although I love her, and she has given me the greatest gift I have ever known... I at times resent that fact that I am constantly reminded that he is hers, too. Honestly, it kills me to admit all this, because I don't ever talk about it to anyone.. but I know how much pain you are going through and I wanted to offer some other insight..
I would advise that you try to stay in contact.. as his grandma. There will come a time, even if they never contact you back, when he will want to know you. I went through this myself with my birth mother's family. Turns out I didn't want to know them, but for years I was plagued with the question of why they didn't care enough to contact me??. Try not to be pushy.. maybe just tell them you'd like photo's and updates of how he's doing. I hope this helps.
One other thing I'd like to add.. it does help a bit when his birth mother and father tell me that they are so happy he's with me.. that they know he is loved and safe. They tell me that my husband and I gave him the gift of a loving, stable home. It makes me feel like they are happy for all of us as a family.