My son is 14 years old and getting constant headache and bellyache he has been diagnosed with migraines but am very concerned tablets not working?

Speak with his doc. Then you need to speak with the doctor who is treating the problem.
I am sorry to. Hear your son is suffering from migraines. Has he had a complete workup yet? He should have multiple things checked when migraines hit at that age, including checking for a pfo in his heart, any masses in his brain, etc. Also, many people miss neck issues that causes constant headaches in youth, so be sure to have his neck checked by someone w/ experience. Also, new study just showed that chewing.