The main reason parents think this, and to be honest people in general about ten relationships, is that at that age people change a hell of a lot in a short time. I mean, I look back at how I was when I was 15/16 (I am now 19), and I'm a completely different person.
Because of this, people grow apart, start to conflict etc and so a reasonable assumption is that it wont last.
I have to wish you luck going to university, some relationships work, some don't, the main reason I know off (happened to myself and others I know in LDR at uni) is jelousy and clingyness/paranoia about cheating etc. Having LDR at university can work, and I have seen people become even closer due to appreciating the time they have together while in this.
Ignore what his parents think, if I dwelled on every little negative thing people said to me about any of my relationships at one time or another, I would have hung myself a long time ago. Just do things how you want to do, **** everyone else. (apologies for lanquage use).