It's generally medium-sized, close-coupled dogs who leap really well after frisbees. A larger dog with a longer back is going to have more trouble with it - their balance seems to be thrown off by leaving the ground like that. I'd say play with the frisbee and the dog a lot, not emphasizing the jumping part. You can roll the frisbee, or throw it low so he doesn't have to leap after it. If he really likes playing with the frisbee, you'll be able to start encouraging him to jump higher after a while.
How old is he? I'm vaguely aware that larger breeds are supposed to not get too much strenuous exercise before a certain age, and in any size dog, puppies and young dogs are more clumsy.