I don't know your circumstances, my friend. I feel so sorry for both of you and your son that you have to be apart and cannot spend every special moment with each other.
I can remember when I was a young child, every time my mother would take me away from my father, it pained me. And my father also said that he missed me. He wished our family was complete but because my mother decided to leave, there's nothing that my father could do. But I always chose to be with my dad.
I know that your situation is different from mine. But the pain and grief of being separated can be similar in some aspects. You miss the affection. You miss the conversation. You miss out on significant events in your son's life. You couldn't be there when he celebrates and you're not around when he needs to be comforted.
I am sure he knows that you love him. And when he's older and whether he gets to read this letter or not, he will still feel the love you have for him. As long as you don't give up on hoping and trying, you will be together and when that time comes, make every single second with him counts and just enjoy the moment with each other. Shower him with your love and affection that only a father knows how to.
Thank you for sharing this with us, my friend. I felt the heaviness and pain in each sentence but I pray that you'll get by and you will be reunited again and form a special bond again just like you did when he was still very little.