My exh abandoned my son a few years ago, unlike your son, mine knew his dad and... was furious at the way his dad behaved towards him in the months before he stopped contact.
Every letter received is received in different ways, most of the times he takes them and leaves them unread in any other place in the house. Other times he reads them and starts asking questions perhaps as he feels left out, but most of the times he is disappointed them with them, as they serve as a confirmation that his dad doesn't care enough for him even to remember what he liked or didn't. In our experience, these letters are written by his dad perhaps to appease his own feelings of guilt (if he has any) but they are not about making DS happy or anything of the sort, they are letters written by his dad for his dad's own sake IYWIM, end of.
At the beginning, I tried to read them to him, now I just let him do as he pleases with them. I realised at some point that I was insisting on DS to keep in touch of his dad out of fear of exh's reactions rather than because it was good for DS.
Considering he has not been in contact with your child, I would suggest to keep the letters for a while, read them to find out if they are suitable for a child (yes, sometimes they have things a child doesn't need to deal with) and see for how long they keep arriving and if they are good for your child.
I wouldn't be surprised if he writes for a few weeks and then disappears again, that would be more devastating for your child than not having contact at all. So, don't hand him the letters just yet.