My boyfriend has a hard time trusting anybody- I'm the only one he does trust. He acknowledges this and wants to find help and help himself, but he's only in high school and his parents don't believe he needs a therapist.

What else can we do if he doesn't have access to a therapist?
What can I do to help him?

In his past, his parents lived in another country and left him and his brother with his Grandma, which made him feel very abandoned. His grandma verbally abused them, and eventually his brother grew issues of his own and shuns everyone. His parents are divorced and there's tension and lies between family members. He makes friends easily, but lost his best friends who now hate him for ridiculous reasons.

He gives all those answers about,
“I can never let my guard down.”...“If I open up I will only get hurt again.”
He honestly feels there isn't anyone he can trust fully because of this life of being lied to and abandoned. This also lead to the paranoia of me leaving him, something that I wouldn't do even if I had to give my life for it.

I have tried my best to comfort and ensure him with my love and support. But what things can I tell him that will actually help his problem?

Thank you all so much for your answers.