The developmental milestones are important for more reasons than just motor abilities, like walking. During the developmental stages, the brain matures and sets the stage for later intellect. For example, the stage where the baby is on his stomach and raises his head at 3 months is not due to strong muscles. He has a reflex that he doesn't control that forces him to raise his head. He needs to spend lots of time on his belly with this reflex in order to gain muscles in his back, shoulders, and neck, so that he can eventually roll over, sit, stand, and walk. The reflex is called the Tonic Labyrinthine Reflex, and perhaps it still needs to be integrated. But that is just motor.
The baby needs to work through this stage enough to also create the neurons in his brain that will help him with sequence and timing. A baby needs to spend time on his back enough to create the neurons for memory, attention, and sensory integration.
My child was diagnosed with learning disabilities at 12, and walked at 14 months - within guidelines, but later than most. At 12, she revisited the stages of development, and that was part of her therapies to raise her IQ by 20 points, and overcome her learning difficulties.
Your boy sounds like he is full of energy, but to be sure that he is developing to his potential, you could play lots of games with him on the floor. Since you have the appointment coming up, I would definitely ask how his muscle tone is, and if the pediatrician has ideas as to why he doesn't walk yet.
Best of luck!
K. Johnson
www.pyramidofpotential.com