All losses aren’t the same. You didn’t say how long ago your bf’s grandmother died but I’m guessing it was fairly recent. It sounds like he lost someone who was like a mother to him and he is deeply grieving.
The first time we go through a meaningful death like this can be incredibly painful. It’s absolutely normal for it to take 3 to 5 years before it is totally accepted. That doesn’t mean that he’ll be miserable like this for years. It does mean that there will be moments when he will be surprised by feelings of loss and sadness even though most of the time he’s doing fine. Loving him means accepting that.
My guess is that he can’t talk to you because you want him to move on too fast. You want his heart and mind to be with you when, for a while at least, they are with grandma. If you can be a supportive and kind presence while he grieves, he may let you in. But if you try to push too hard, he will need to distance from you to do his grief work.
I hope there is someone who can help him sort out his feelings and come to terms with the loss. If you know of any adult he is even a little close to, it would be a kindness to let that person know that your bf needs some comfort and support.
Meanwhile, do a little reading up on how to help someone who has lost a much loved parent or parent-figure. If you can be helpful instead of demanding and if you can put his needs first for awhile, he may be able to come back to you.
I wish you well.