My son is 5 years. He developed stone in the duct of left submandibular gland 2 times. Gone without medications or surgery. Please what can I do?

See your doctor. You should be seeing an ENT specialist for evaluation, treatment, and prevention.
Salivary stones. Salivary stones are not very common in children. Did the doctors actually see a stone on imaging or on exam? Generally an ENT could perform an exam and with an imaging study determine the problem. He may do well with a "sialendoscopy" procedure. Or maybe he just needs to remain better hydrated (dehydration predisposes to salivary issues). Very last resort would be removing the gland.