Year 4 wife here. We also have two kids, if that makes a difference. Also from an international school, which gets crapped on in this sub, but hear me out!
What worked: honestly, as cheesy as it sounds, the thing that works most is giving 110% to each other. Sit down and have a very frank conversation about what expectations are on both sides and what concerns each of you have. Think about worst-case scenarios and how you'll deal with them. Also--super important--make time for each other and make time for yourself. My student and I had gym dates once a week and got a sitter, and it really helped us stay sane and close). Also, have sex even if you're tired. Treat yo self, even if it's only a little by and you can barely afford it. Empathize with your student while also helping them see your side of the story.
Another thing that works: when your student encourages your goals/interests, just like you're doing with them. In the time he's been in school, I've gotten an MBA and started a freelancing business (and squeezed out two kids, but that's a whole different story), and it's been so much easier knowing I have his blessing than feeling like I'm somehow not supporting his goal if I'm nurturing mine at the same time. I know this particular situation doesn't work for everyone (especially if you work in a physical location and your student needs to move somewhere else), but definitely be mindful of it and how you'll negotiate those challenges.
Another thing that works: planning and organization. I'm talking schedules, budgets, chore charts (silly but effective), and whatever other systems you can establish for keeping your time and household running smoothly. This is especially important if you have kids, but for anyone it can prevent arguments and get everyone's expectations in line.
What doesn't work: playing the "whose life is harder" game. Sure, there will be times when you feel like you never see your student, when you feel lonely, when you feel like they don't care about you, when you feel like you're doing all the work (such as helping them prep for their day, child care, going to work to pay their way, etc.). But you also have to acknowledge and understand what your student is enduring right now: basically a gauntlet of constant studying, sleep deprivation, stress over their grades/sense of self worth/future, and being shit on by any number of people. When we stopped playing that game and actually tried to help each other (that whole giving 110% thing), we both felt more valued and supported.
How did I help him understand the time commitment? I'm not sure I understand that question. Both you and your student should definitely know the time commitment before they sign up for something like this. Talk to as many people as possible (alumni, friends in the medical field, Reddit) to get a realistic picture of how the next few years of your life are going to look. It's definitely not easy, and we've definitely had bad days, and it took us a long time to get into a rhythm. But as long as you can empathize with each other and stay organized, med school isn't too unbearable for an SO. Good luck to you.