I don't know how old your son is? That makes a big difference I think, as well as how he's doing emotionally with his adoption or situation surrounding his bfamily. Even if he doesn't remember bgrandma, is he aware that she exists?
I have 2 kids that don't remember their bparents and 2 that do (same bparents/sibling group). The 2 that don't remember ar 5 & 6 and the 2 that do remember a bit are 7 & 8.
The ones that remember are probably at a point where they want to see a picture or have a letter just to be assured that the bparents are okay and they are not dead. The ones that don't remember have some curiosity but not much. My 5 year old is still coming to terms with what adoption means and isn't quite there yet. He is very loyal to me "you are my momma, not HER" and it's a challenge right now for him to understand everything. He'll get there though!
However, if I should receive anything from the bparents(letter not answered as of yet...), provided that the info is not harmful or disrespectful, I would likely share this information with them. I'm of the opinion that it's better to share information now rather than later because I believe in the long run, honesty is the best policy. I don't want my kids to say to me "well, my bmom wrote to me but YOU wouldn't let me have her letters because you hated her" etc. I believe it's their information to have and not my right to keep it from them. Of course, as their parent, I have discretion over that information, but want to put their best interests first.
But again, that's dealing with my kids. If your son isn't in a place where you think he would accept the information readily, then maybe it's not the right time.
I'm not sure what makes you uncomfortable, but you know you always have the option to share a few parts of the letter and not all of it. There's nothing wrong with sharing a bit at a time to test the waters on his reaction and feelings and move from there. If he's young, you can share things like "I heard from your bgrandma today and she's thinking of you. She hopes you are growing big and strong" etc. Or even prior to sharing the letters, you might talk about her in general. Explain who she is and that eventhough she doesn't see him, she loves him. This might help you decide when the right time to share the letters will be.
Hope that helps?
If not, feel free to ask more. And hopefully others will have some suggestions too.