Hey, so this is a sort of different situation, but I'll give you some advice.

There's no sure-fire way of changing someone's mind about something like that, however, it's not completely unfeasible to discuss it more to the point where he might.

Make sure he knows how you feel, even if it might hurt more to put all your feelings out there for him to maybe still not change his mind. Think for a few days, then tell him you've thought about it, and you have decided that no matter what, you're on the side of staying together, and you're able to make that work for X number of reasons. Do some research into careers, moving state, whatever, and then come back with an informed opinion, e.g. 'one day, in a year or two when I've got some experience with this job under my belt, and a little more money in my bank, or you have, it's possible I can move to your state and work for this company in my field, or you can come here. Or, we can move together to another state where there are a lot of opportunities' and maybe even show your research. "We can use our vacation time to see eachother- we can go somewhere different together, or one of us can come and visit the other in their state when they're working. It may even be exciting to have some sort of reason to travel around and see the country/the world a bit better.".

If there's a possibility of weekend trips, even better! And that means you won't be distracting eachother during the week - trust me, sometimes it's actually nice to not have someone else to think about when you're tied down with work and only have time to work, eat and sleep, which is unfortunately very common for many graduate jobs these days.

Say your reasoning behind it, ultimately, you may already recognise that it may not work out for either of you. That's ok. But is there any harm in trying? Probably not. You're both young, there's no rush to finding a life partner and if you already feel you've found yours, you may as well give it a go at trying to make it work out. Say this to him - say you ultimately recognise distance is hard and sometimes it's unfeasible, but you're willing to try it out to see, and you'd feel dissatisfied if you just ended things and in your mind all the time was a constant "what if".

A little backstory from my own side, which is quite different to yours but he had similar doubts in the first place so it may give you a little bit of hope:

My boyfriend and I met while we were travelling, we met at a hostel in Istanbul. We went around seeing the sights for a few days, we hooked up one evening, then we were going separate ways, and we had eachother's contact details and a plan to meet up when he came to London later on in his trip, but no other thoughts.

I spent the rest of my own trip thinking about him, even though at this point I barely knew him. When we managed to get wifi, we spent a long time chatting and snapchatting and flirting. We decided he was going to stay with me when he was in London. We spent the evening and the night together, then unfortunately he went back to Canada and I was left in the UK.

We continued chatting. We started skyping. Luckily for me a month later an opportunity came up for me to go to a conference in Toronto. I tentatively suggested this possibility to him, and he jumped on the opportunity too, and I spent a week in Toronto staying with him there.

While I was there, we decided to discuss "us". I was open to the idea of a relationship, and to be honest more on the side of doing a long distance thing, but sort of remained neutral. He decided he didn't want a long distance thing, but was really enjoying the "thing" we had. We were both a little confused, but we continued to enjoy our week together and we even started making further plans to travel together (something we both love doing).

We planned a trip to Rio together after I got back from Canada. A few days later, he asked me if I felt like we were in a relationship, and he said he'd like it to be a relationship (apparently booking a trip and making plans prompted this, as well as him seeing a photo of me with a male friend with his arm round me and he realised he was more jealous than he perhaps should have been).

We've now been together almost 6 months. There have been times when he's doubted it, and we nearly broke up at one point, but we're both able to make plans together still to see eachother, and we've discussed how we potentially could close the distance, and we've decided that it's worth it because we like eachother so much.

The key for us has been the ability to make plans so we can see eachother. If this is unrealistic for you, it may not work, as it seems your boyfriend inherently values time physically together rather than you being emotionally present. The reason I included my own story is to tell you how you can recover when one person doubts it, and all the factors you may have to consider.