I get that, but probate rules don't just say if you are owed money you can keep their shit. And likely whatever is in the house, considering they weren't even paying the rent, probably isn't worth the time it would take to sell it. Based on my experience his son will eat the $4k, maybe even have to pay to have someone clean the crap out of the house (or do it himself), and move on.
And frankly, I'm not sure where this rental is, but only being behind $4k during an eviction moratorium is probably doing pretty good all things considered. If renters have decided not to pay for 18+ months at this point, he's either wildly undercharging the rent or they didn't go too many months without paying.