It might be mean to tell a guy that you're unsure of how your parents might feel dating someone who doesn't make as much as you or doesn't have his GED. But also, sometimes people need a fire lit under their ass to get things done.
The argument isn't really about money at all. It's about goals and what you'd want if you took the relationship to the next level. You think this guy can do better and you want the best for him and for your relationship. You've already alluded to this: the guy is in his 30s and doesn't have his GED. What does that say to you about him? If it takes this much for a guy to push through getting his GED, does your crystal ball put up red flags for the future? Is the little voice in the back of your head whispering in your ear? A hard knock life is a very troublesome thing to overcome, but eventually you either do or you don't.
What does he want? Is he comfortable with having a secure job a house and a car? Does he want more?
Love is grand, but it doesn't always hold a relationship together. Say he gets his GED and you move in together, and he's comfortable with where he is. Will you then be bothered if he doesn't want to continue with a traditional or vocational education? What are his goals? What does he want to be when he grows up? What are your goals? He may already be leaps and bounds above where he came from by owning a house and a car.
Are you comfortable with that or do you feel like this guy has settled? It may not really bother you now, but in a few years you might start to really resent it. And you have a perfectly valid concern- if he doesn't get his GED then he wont have anything to fall back on if he's fired or layed off. There aren't that many jobs available these days for those without at least a GED, and probably fewer in the future.