I guess my first question is...Why is he not seeing his Father on a regular visitation basis? Such as Every other Christmas, Easter, Spring Break, etc. It is very important that he be able to know when he is going to be able to see his Dad. The Dad needs to step up to this responsibility. It should not be all up to you.
I have 6 children. I have a 13 year old too. The best thing I feel you can do is to make sure you keep him on a short rope without letting him know he is on a short rope. Have parties at your own house so you can get to know his friends. Ask him if he wants to invite a friend over often...for dinner or whatever. Take trips to the coast and have him take a friend.
Teenagers start to seek out the wrong kind of influences when they are sad and feel abandoned. They get real angry inside even though they are only showing sadness on the outside...if that. If you allow him freedom to go elsewhere, you have no idea what influences he is putting himself around. BUT...DO NOT LET HIM KNOW THAT YOU ARE MONITORING HIM OR HIS FRIENDS. Just GET INVOLVED in his life.
At this age and older, it is normal for them to want to break apart from family and want to 'hang' with their friends. So, if you have the friends a part of your life too, this will make it all the more closer for you and he to discuss any issues as they might come up.
Make life fun. You will be taking the back seat as far as your "friendship" with your son because he will be wanting friends more. He loves you!!! Just stay in the picture though.
I sure hope this helps! It is really important for you to not be the one to always get him to his Dad. If his Dad wanted to see him, he'd make the effort himself. This may hurt your son, but that is just a road that has been chosen for him by the decisions made. Sometimes you can't always cushion the obvious.
Take care!