How can I find out my sons adopted name

If this was a closed adoption you can't. If you are looking for your son and he would be over 18 by this time, there are several adoption "reunion" sites where you can post the details of his birth and if he is looking for you, you may reunite.

Or you can talk to the adoption agency you used to see if he has left information with them for you.

If he hasn't, you might be able to leave your contact infowuth them in case he ever looks for you.