I can understand how difficult it is for you. My sons first wife left him with his little boy, moved away, wouldnt accept any contact from us with our grandson and then after a year moved abroad and we have had no contact since then. I am now a widow and can only hope that when he is an adult he will try to get in touch. So my suggestions are ; perhaps you could get a birthday card for each year and could y ou put a photo of yourselves that you take on that day too? If you have a particular hobby, gardening or fishing or whatever a picture of you involved in that would be good. If he gets in touch then he will be able to see where you lived and what you were doing and how you looked over the years and it would show him that you never forgot him.
Also there could sadly be the possibility that he may not think to look for y ou at 18 but it could be many years before something sets him onto wanting to trace his origins and birth family. So that even if he only found out this information in his 60's , he could see what you were like and have that knowledge that you never forgot him.
Again from a practical point of view, perhaps you could put a small sum of money on his birthday in an account , with a building society or bank in your own name, but which you know is meant for him, and you could tell your son where it is for the future.
My own son is adopted , and so I can see this from both sides . We have never made a secret of this fact and his name is Daniel , which means "the chosen one" and when he was grown up we were able to help him find out what he wanted to about his birth family. I do think photographs over the years will be treasured , if and when he gets in touch, and in any case you will be able to look back over the years and remind yourself what you were doing at that time. Or you could get a small photo album and each year put the picture into that. Again when he comes to find his past, it will show him that he was still always part of your family and was never forgotten. Best wishes