15 Year Old Doesn't Seem to Have a Close Friend Anymore

You can't encourage him without making him feel like there is something wrong with him, unles it's something HE wants to change. It's great that he doesn't seem depressed, keep talking to him and paying attention. If he is content with the social life he has, you need to accept it. There are plenty of introverts who get by with what you may consider minimal social interaction and are fine with it. Don't create the sense that you think something is missing in his life, it's too much pressure. Follow his lead.

Do you ever just chat with him?
Not lecturing or grilling him or telling him things or talking at him... but just chatting? about whatever?
Try that.

At this age, some kids don't even want to be at home or with their parents hanging out. And they are into really bad stuff already and have bad friends and have lots of negative drama with friends. So be glad at least, that he does not seem this way.
He seems, at ease.... comfortable with what he does or is doing.
And at school? Happy there? If so, don't worry.

This age is hard. I remember being that age. Good grief, SO many kids, getting into bad stuff or opposite sex stuff... and getting into cliques at school etc. Not all positive either. Some of it can be quite toxic.
Many kids at this age are also just followers.... just copy cats doing what others are doing, good or bad. So be glad, if your son is HIMSELF and knows, himself.... and follows his own instincts. And is not, a copy cat follower type kid.

I had friends at that age. All kinds of friends. But, I KNEW... which ones I could not trust with a 10 foot pole. And sometimes, there is just not many kids.... whom I even wanted to know or be friends with. The pool of kids to "choose" from as a friend, was not much.
So think about that too.

Maybe your son is just himself... and even if he is shy.. so what.
MANY great successful people, are shy.
What are your son's.... interests? Nurture that.
And just chat with him, so he can express himself and tell you things about his life. In a normal way.
So that you both will always have a good rapport and as he gets older... and does not drift away, from his parents either. Even if he gets older.
So going back, don't make him feel like something is wrong with him. Because if you do, he WILL.... drift away, from you or Dad. And that is not good.
He should not feel self-conscious in his own home... about whether or not he has a good friend or not.

At 15, he may have drifted away from his friends because they were getting into things he didn't want to get in to (drinking/drugs/girls). As long as he's okay, I wouldn't push anything. Follow his lead.

He's involved with the golf team and goes to a church youth group every week. Is he keeping up his grades? That may be all the social contact he needs and wants at this time. If his grades are still OK and he interacts with you, I would not worry about it.

You could just ask him a couple of questions, like, "I notice you're not hanging out with you friends any more. Why not? How do you feel about that?" etc.

His answers might reassure you, or help you lead him in some direction, like a group to join, etc.

my kids are very young but i remeber my friend change in my freshman year. while the parents were aware my neighborhood friends were testing the boundries, exploring with sex and drugs, sneaking out, drinking, finding themselves. I backed off and cut off contact with them. I am a goodie goddie and did not want to find myself in trouble. so I carefuly searched for new friends. I took a meyers briggs test and other personality tests. it helped me alot to realize who i was. i also am at first a very shy person. although i have no trouble making aquantiences i like my alone time. i get overwhelmed and very tired after shopping. like overload fro my brain. give him space you never know if his friends are on a high speed run down the "wrong" path. also these friends were chruch friends too as well and non church friend.s

Obviously he is content in his life. So, I would let him be. He is 15 and can handle his own social life.