Need help - My brother's company website redirects to a parking domain or CPA offer

I need help, guys. Since early April this year, my brother's company website would randomly redirect to a parking domain or a CPA offer when I click on the URL on Google based on his main keyword (it's a local food product search term).

The ratio of being taken to his website vs being taken to a parking domain or CPA offer is around 9:1. His ranking steadily dropped from number 3 to 32 on local Google search in April. His website used to be in the top 5 ranking since it was established in 2010.

His website codes, WP theme and plugins are clean when I checked them. Does anyone know what might cause this redirection problem and how to resolve it?