Boomerang the son is not the property owner. He does not have the legal authority to let someone onto the property without a warrant anymore than you can let the police onto my property to look for suspected wrong doings.
The laws against unreasonable search and seizure are there, by the wisdom of our founding Fathers, for just this reason.
This is an issue of principals and property rights. Think big picture for a moment.
Some yahoo with a badge around his neck shows up at your doorstep and threatens you with arrest if you make him come back with a warrant and he finds something he claims is illegal, in this case shelter for the animals. So what is going to prevent him from arresting you if you allow him on the property without the warrant? Nothing.
NEVER EVER EVER talk to anyone in law enforcement without a lawyer. Nothing you say can help you, anything you say can be used against you. " We have shelter for the animals" was spoken by one of the people. If that turns out to be a lie, add the charge of lying to a police officer to the list of charges. A warrant also limits the scope of the search. If someone allowed a cop, ASSUMING THIS SPCA INVESTIGATOR IS SWORN IN as a "SPECIAL" in Rockingham County, onto the property, everything in plain sight can and will be used against you. " Your Honor, we were allowed onto the property by the son of the owner and while looking for the existence of proper shelter for their horses, we saw in plain sight (insert something slightly illegal here) and in seeing that (illegal something) we had probable cause to search the property and we found (more questionable stuff).
Anything they see that is outside of the scope of the warrant is going to be hard to be used against you in court. Anything they see if you let them in is going to be used to hang you.
I think I am going to send an email over to the Sheriff of Rockingham county asking if the SPCA investigators are indeed sworn as specials.