Idk how hard of hearing your dad is but this is what I use and it is LOUD.

LIOTOIN Wireless Caregiver Pager Call Button Nurse Alert System Call Bell for Home/Elderly/Patients/Disabled 3 Transmitters 3 Plugin Receivers (600+ft Operating Range) https://www.amazon.com/dp/B083XPDPNR/ref=cm_sw_r_cp_api_glt_i_C5C6JS72XVWMYSD7BEVJ?_encoding=UTF8&psc=1

I have the opposite problem of your dad in which I am very sensitive to noise, but this thing is LOUD. I can easily hear it from downstairs whenever my dad presses the button. Put two of these things in your dad’s room and he will wake up for sure.