I feel like our 20s are a confusing time - it's full of excitement and new experiences but the price of that is upheavals and uncertainty. It's hard to know whether in 5 years time, you'll be in the same job, city or even, country.
I feel like I can relate; I'm in a new-ish relationship (almost 5 months) and am currently looking to find a new job in another city (~2hrs away). I'm moving because there are more opportunities and most of my good friends live there. My SO is also at the cusp of a career change and he has been applying for anything and everything. It's a difficult situation - things are going well in our relationship and we can both see ourselves in this for the long term however, our careers are important to us and we both feel like our 20s/early 30s are the time to explore the world in general.
I'm not with my SO purely for entertainment until my career takes me elsewhere. Neither is he with me. We have just unfortunately met each other at a time when our future is still up in the air.
We spoke about it recently and have decided that bearing an international move, we are going to give long distance a go once one of use (or both of us) move however, we are not going to hold ourselves back in pursuing our career dreams for one another (as this relationship is still new). I told my BF that I'm willing to work at this relationship (weekend travel visits, etc) once I move but I'm not willing to stay in the same city for him. He told me that he too, is willing to work on long distance however, if he had a great offer to move internationally, unfortunately he will take it and that will be the end of us (though this is v.hypothetical). It sucked to hear and we were both really direct about it but it had to be done.
I suggest you talk to your bf and discuss all and any future scenarios that have a high likelyhood of happening. See if he's willing to give it a go. You both might find that LDR doesn't work for either of you but at least you know for now that he's open to it. I think your BF is being vague about it because a) he still doesn't know yet what he's doing. He might not feel like there's a need to discuss (and argue) about something that might not even happen and then you'd be hurt for nothing and b) he knows, due to your insecurities that it will be a highly emotive conversation and is scared of the potential of you breaking down/crying (i think this is also why he didn't tell you about him applying for the fellowship in person - but over text).
You need to calm yourself down and think through what you want from this relationship when and if one of you moves. Maybe look into all the travel necessities to keep it going (do you see each other every weekend or every other weekend). And then you talk to your bf and listen and try to not let your emotions get in the way of the conversation. Come to an agreement of how far of a distance you both are willing to overcome (2hr drive seeing each other every weekend vs 2hr flight seeing each other everymonth vs 10hr flight seeing each other every 3-6months) and then decide if his terms meet yours and whether or not you are willing to stay with him knowing that the relationship might end.