Sure, one could alter the tone/EQ settings and perhaps achieve some marginal benefit. However, to truly affect the situation, the source needs to be closer to the listener.
Your Dad needs to experience more direct sound, and less reflected sound. The simplest manner to achieve this is sit closer, done. The speakers that come built in to TVs ... with a few exceptions, have poor directional characteristics and much of their energy excites the plastic enclosure body, .. thus making intelligibility and articulation even worse.
Ideally, get a pair of outboard powered speakers, and place them on each side of the TV. You could place them somewhat closer to him, but I'm guessing simply getting some modest speakers will help a lot over the built-ins.
Something like these
would be fine. The TV will have connectivity options/variable line outs that will be adjusted via the normal volume control of the TV. You could try even cheaper computer speakers, but something like these would be better. Any big music retailers in your area will sell items like these. Small, active/powered desktop speakers. Places like Sam Ash, Guitar Center, etc, will have an assortment. The online sources like Musicians Friend, Sweetwater, Parts Express, Amazon will also have whatever you opt for. When looking online, these are found typically in the recording areas, under active monitors.
Other options would be incorporating some speaker/amps you already have. Also, if considering the cheapest route, cheap computer speakers, your local Frys or equivalent would suffice.
Get the source closer to the listener, the levels can be the same for the listener, yet reduced through the remainder of the house.
Best of luck.