Why don't you trust him? Have you ever met the best friend or seen them interact in the flesh?
2 of my 3 best friends are guys. One I dated for 8 months in highschool, around 8 years ago. We are extremely close - we share everything. But for us, the fact we dated once a VERY LONG TIME AGO is irrelevent. We used to live 3000km apart until my 2 best guy friends moved to the same city that I moved to vet school for. My SO, who I had been with for 1.5 years at the time, was really apprehensive about him moving here - he doesn't really have friends of the opposite gender, let alone close ones, let alone exes. But he was a good sport and gave it a go - because he knew my besties are not negotiable.
They are now best friends. Sometimes I think they like each other more than either like me!!! 😛
Sometimes in life I just think you have to accept that sometimes your feelings DO mean less than someone elses. I think it would be genuinely horrible if you told him he could not go see his best friend and potentially say goodbye for good because you feel insecure. For me, a best friend dying does mean more than a problem in a relationship, especially one only 1.5 years old. And if your going to make a big deal about me wanting to spend time with a dying friend, your not going to be around much longer than that 1.5 years.
In fact, I don't think you should even say something to him about your insecurities if he does go. Unless he's cheated on you in the past, thats your problem, not his, and this is a hard enough time for him as it is. I do think you need to suck it up and deal with that on your own, as harsh as that sounds.
So he shouldn't see his potentially dying ex-girlfriend who is his best friend because of your own personal insecurities? That is selfish as ****. Its not "coming second" its compromising - which is extremely important in relationships to me. No-one can come first all the time.