Is the executor the only person who can challenge a will? Absolutely not. That would mean heirs or those disinherited would have no rights. The likelihood of a challenge being successful is a different matter but to answer your specific question a will can be challenged by an interested party which would include named beneficiaries and heirs-at-law.
This is not legal advice nor intended to create an attorney-client relationship. The information provided here is informational in nature only. This attorney may not be licensed in the jurisdiction which you have a question about so the answer could be only general in nature. Visit Steve Zelinger's website: http://www.stevenzelinger.com/
Any person named in a prior will as a beneficiary or in the current will or any heir at law can certainly challenge the will. Consult a probate attorney on practices will contests now while he is alive as to any actions you need to take as there are ways to block the new will from being accepted for filing and to litigate the matter. Once the will is admitted, the executor will have access to all of his assets and you are then stuck appealing the matter, but if you can block initial probate of the will then you have more leverage. I note the statute of limitations for these sort of cases is very short so do not delay at all.
No-the executor is not the only one who can contest a will.
The most common reasons to contest are undue influence or incompetence.
If a will is changed because of those reasons-it can be contested.
The answer given does not imply that an attorney-client relationship has been established and your best course of action is to have legal representation in this matter.