Why did my partner said I was his "darkest-secret"? He says I know more about him than his family (mom) does. Please read before replying 😇?

Ever since my guy best friend and I did what we did (personal) lol. He's been opening up to me lately. Like he's been telling me things that I would never thought I'd hear. Whether it happened in the past or present. We've dated in the past, had intimacy and etc. Our families know each other and our moms are best friends. He's even told me when he comes back from deployment he wants to get me a ring. We've talked about our futures, possibly having kids (not sure together) he said he would come to my graduation because I'm a chef at school. But yet he says we're just friends. Idk what's going on, he keeps certain things to himself. He's told his friends about me, We stay on the phone sometimes hours and hours, singing or just making conversation. He's very supportive and helps me whenever I ask him. But it's very confusing. Idk if he wants me in his future or not. What do you think ? Are we just friends? Please help.