The registered will bequeathing the self acquired property in favor of one son only by the testator, in general, cannot be challenged or even if there is a challenge it cannot be maintainable owing to the nature i.e., registered will. But remember even the registered will can be cancelled by the testator at the last moment or at a later stage, hence better to keep the registered will as a confidential document.
The medical certificate about his sound mental health will support to prove the genuineness of the will.