Your question is essentially a variation on a debate in the autonomy literature. The question that arises there is the relationship between autonomy and free choices. The two examples most common in the literature are prostitution and burqas. The question in these instances and yours is this:
To what extent is autonomy to be located in the immediate will of an individual and to what extent is it located in having a well-ordered will. And then to what extent do we have a right to intervene in autonomous but poor choices.
Clearly, cleaning the house is against his immediate will. But then the question is if it is against his-all-things-considered will or his-all-things-considered good.
Here, in the case a hoarder, part of the question will be whether you consider his current cognitive function sufficiently impaired to decide whether or not can engage in autonomous acts of will. If you think he cannot engage in autonomous acts of will, then it is merely a question of whether it is right or not to clean his house.
If you think he can engage in autonomous acts of will, then the question is whether the situation warrants a paternalistic response.
But before you clean his house, you should consult with a counselor specialized in hoarding. Several people in my wife's family are hoarders. And from that I've learned a little bit about hoarding. Hoarding is a type of executive function disorder -- meaning that the person has a hard time deciding what is valuable and worthless. Moreover, it is often something that has an onset at some point in life and can get amped up by depression.
To put it another way, hoarding things is a symptom -- not the root issue itself. But then cleaning things for a hoarder doesn't work because the function problem remains.
Now, all that being said, if the house has become unsanitary, this all seems minor...