My background: ex-RAF, ex-Recruiter, boyfriend ex-military and was serving when we met.
Worse case scenario first, be prepared to support your boyfriend if he doesn't make it into the Army or, subsequently, the Commandos, either through selection or during training; there are a lot of hurdles to get through and there is always a risk of injury that I know has stopped people's progress entirely.
Remember, people do have relationships with one partner in the military. Patience and communication is key! You need to talk to him and find out more about what it involves. There will be times when he won't be allowed communication, or he'll be too busy and tired to do so. There will be things that he's not allowed to tell you. As well as his normal role, there'll be extra duties he will take on, some voluntary, some not; further training courses, guard duty, parades, extracurricular to boost his profile and promotion prospects, sports and expeditions just for fun. Things will happen at short notice, he won't necessarily be allowed leave when he wants and could work weekends so yes, he will miss events.
He's got a lot of other things to think about, so maybe he's hoping that the relationship is fine and it's one less worry for him. Ultimately, you'll be able to visit each other and it's worth you going to his base and seeing some of his life there. You may even meet others in the same situation as you. End of the day, you could as easily split up if he wasn't going into the Army and you were living together as him joining the military. Friends and family are right; make your plans and go with the flow, as there's no knowing what might with his future or for the pair of you.
Happy to chat further or PM me!