There won't be anything about rent in the will because rent relates to a tenancy, which is governed by the Residential Tenancies Act ("RTA") (or the VIC equivalent - I think it has the same name).
Estates therefore have nothing to do with rent. If there was a tenancy agreement in place prior to death, then under the RTA, the estate becomes the landlord immediately upon the death of the father. In this case, the son would effectively become the landlord because he's the executor and you therefore have a situation where the son is both landlord and tenant. The obvious problem here, is that the tenancy agreement remains enforcable under the RTA, but the two parties to the contract are now the same person, thus there is a clear conflict. Whether or not it is a conflict that is resolved by law is another question, because I don't think the RTA covers this scenario. The most important point though, is that the son can terminate the agreement all on his own because he is the executor.
If there was no tenancy agreement in place prior to death, then clearly the father didn't wish to charge the son rent and his wishes should be respected - but only as far as practicable. If the property takes an excessive time to sell, then at some point, it must be accepted that the estate is not financially fulfilling it's potential. This then becomes an issue with the estate itself and how it's being managed. So in this case, the issue is not about rent, but potential earnings to the estate not being realised and as such, the siblings would have a right to take action against the son to hold him liable for the loss of potential income to the estate. In other words, the claim would be that the estate is being mismanaged.
One thing the siblings need to be careful about, is that while the son may be benefiting from living rent free, it appears that he is also shouldering all expenses relating to the property. If the siblings demand rent, then they must also shoulder their share of all expenses relating to the maintenance of the property. Depending on the condition of the property, the son might easily be able to come up with tens of thousands in repairs just to teach them a lesson.
So common sense should prevail here. As long as no one is intentionally holding up the sale of the property, then I think all parties should let this issue go. Squabbles like this could easily lead to a lengthy legal fight in the future which could be costly to all involved.
Easiest solution? See if everyone can agree on a reasonable timeframe for the sale and if the property is not sold by that date, then the "potential earnings" issue will be addressed at that time. This can be done with a tenancy agreement or some other form of contract requiring the occupant (son or otherwise) contributing financially to the estate for having exclusive use of the property. But don't forget that it's a two way street - any such agreement must also require the other siblings to contribute to the costs of maintaining the property, just like any other landlord would have to.