Well, not being able to equalize at 12 years old seems quite odd to me. Usually children learn spontaneous to equalize around 3-4 years, as soon as they are asked to play the game of retrieving toys form the bottom of the pool.
Their eardrums are very delicate, and they start aching at just 1- 1.5 m depth.
So, if at 12 years he is not able to equalize spontaneously, there is a serious risk that there is some medical problem, and you should have him evaluated by an ENT expert of diving and equalization.
Only after having assessed the lack of any physiological problem you can try some exercise in the pool.
I would avoid forcing equalization without a medical exam.