If the will includes the standard phrase leaving the estate to the executor "with power to postpone sale", yes the executor can do that. However, the executor is required to act in the best interests of the estate and the rental income should be accounted to the estate. Frankly, renting a property in the current climate is probably in the interests of the estate if it would be unlikely to see.
As for remedies generally, unfortunately, there are few. Unlike a trust, a beneficiary under a will merely has a "chose in action" to have the estate duly administered, he does not have a beneficial interest in the estate until such time as it has been completed. However, he CAN ask the executor to provide accounts under s25 Administration of Estates Act 1925, by issuing a summons through the probate registry - this is unlikely to be granted until a year after death.
If the executor is acting fraudulently or in conflict, it is possible to apply to have him removed as executor - but you will need convincing evidence to persuade a court that this is the case.