Can an executor who is also a beneficiary contest a will he is executing?

Generally, a will contest is filed when notice has been given that someone has petitioned the court to have the will allowed and appoint an executor. An executor is not appointed until the will is allowed. If there is some dispute as to the validity of the will an executor will not be appointed until the court makes a decision whether or not to allow the will.

So to answer your question, until the will is allowed by the court there is no executor. After the will is allowed and an executor is appointed, it's too late to contest the will.

Sometimes information becomes available after the will has been allowed such as a later will found. That is another category of court action.