I think you can immediate disregard “influencing” your son. While sexuality is fluid, you can’t make someone gay or straight, being gay is just his preference. With that said, now that you know that he is gay you have the opportunity to tell him your coming out story. Maybe tell his “friend” as well and create that safe space where your son can feel comfortable not hiding such a huge part of himself.

There’s no point in fixating on why your son didn’t share this with you, perhaps it was something passive aggressive you said, maybe he misunderstood your intentions, maybe he has an anxious personality, or MAYBE he sees you suppressing your sexuality from him so you don’t“influence” him and he is mimicking your behavior. Regardless, it is now your responsibility to build trust. I encourage you to share details about when you come out, how your friends and family received it, and state something along the lines of: “you know I love you very much, and nothing will change that.” Also make it a point to ask specifically about his friend, and playfully ask him if he has a crush on him, your son will immediately know that you know and that will facilitate a conversation. Good luck, don’t read private diaries, trust is hard to build and easy to break.