Now that you say there's a third person involved and you are all friends, it puts a new light on it. I'm certainly no counselor, and I have never been in a situation like you describe. If I can read between the lines a bit, I'm thinking that you may feel that you are losing a valuable friend, you think it may be because he doesn't want to "come out" to you, and you aren't sure how to let him know that his lifestyle would have no effect on your being friends with him.
If he is making up these excuses often to exclude you from being with him, you may at some point want to mention to him that sometimes you feel that he is trying to avoid you, or not include you in his plans, and that it sort of hurts you at times because you really value his friendship. (Something along that line.) That would possibly open the line of communication for him, and how he responds to you would be your signal as to how to proceed.
You might want to stress to him, if he still seems reluctant to talk, and seems evasive, that if there is some reason that he doesn't want to talk to you about anything, let him know that no matter what it may be, no matter what is on his mind, it would not possibly affect your friendship. Just let him know that you value him, and if there is something you could help him with, or something he would like to talk over with you, you are there for him.
I can't tell you that you should bring up your suspicion yourself, but if he should sort of start the topic, I think it would be very helpful to easing his fears that you could admit to him that you have suspected this, but didn't want to say anything because you value his friendship, and didn't know how to tell him that it would make no difference in your friendship, that you didn't want to lose him.
Maybe the relationship among the three of you could then continue on a new plane, with understanding all around.
Again, I'm not a trained counselor at all, these are just my gut feelings as to how to handle it. Since it affects you this way, and involves a 3rd person, I would be in favor of you talking to your friend as I've outlined above.
I was a public school teacher for many years, and have dealt with many situation of students in their teens. There's a certain wisdom that comes from that over the years, and that's what I'm calling on in my advice to you.
There is a problem here, it is bothering you, and for good reason now that I know there's a 3rd person involved, and you are feeling excluded by them. Hopefully you can do something about it before it eats you up, or you lose a friendship needlessly because he doesn't know how you'd react. That would be the worst result of all.
Best of luck to you...good friendships are valuable. I hope you don't lose his.
And thank you for your put-down of B747-437B. It's none of his @#!!@#
Those who want to do something will find a way; those who don't will find an excuse.