Firstly, I am so sorry that you have lost your grandson to adoption.
However, having worked for an adoption team, keeping a memory box was something that the social workers always recommended. Should your grandson want to get in touch in the future it could really help him too.
I think it will give you some degree of closure and will enable you to regain some control / input for the future. Obviously, his adoption is entirely beyond your control, but in this small, but important, way you can demonstrate that he means something to you. Also from his perspective, he may take great comfort from your memory box.
I would suggest you copy any photos you have of him and include them in the box, along with photos of you, your son and any other immediate family and do this every year, perhaps on his birthday with a card and a ‘newsy’ letter talking about what’s happening in your family and hopes for his, including things like, “I expect this will be your first day at school, I have been thinking about you and wondering how you are getting on”. I really believe that this will allow you to process what is happening and may be really important for him in the future.
Perhaps if your son had any items of clothing, such as baby shoes this could be something that could be included. Maybe if you did a joint memory box with your son it may also help him to come to terms with the adoption. Wishing you peace 💐