Here's some quotes from posts on the Internet:
"We were there in the beginning of April (2012). It was beautiful, as it was snowing on the day we arrived there. However, very few parts of the park were open, and almost all the hiking trails were closed because of snow. I recommend going there later in the spring when most trails are open."
"It's snowing up there right now (March 2012), apparently, and the road to Crescent Meadow will certainly be closed. If snow is as forecast (3-4 feet), then most trails will be closed as well. Still, if you call the number above close to when you are planning to travel, you may find that a few trails out of Lodgepole are open (and General Sherman tree area will almost certainly be open)."
"Call the Lodgepole visitor center for current conditions: 559-565-4436"
Note: I did not verify the phone number, above.
Weather info for Sequoia NP from National Parks Website:
Seasonal Averages Mar
Avg High 48
Avg Low 26
Avg Precipitation (in.) 6.63
Avg Snowfall (in.) 40.0
Avg Snow Depth (in.) 39.0
You may wish to consider Death Valley, Zion, Lake Mead and Lake Powell National Park/Recreation Areas as other destinations, where it will still be very cool, but most likely dry, with no snow.
Plus, if you visit several of these National Parks, you will save money by purchasing an "Annual Pass", or even more, if you Dad has a lifetime "Senior Pass". See link for details: U.S. National Park Service, America the Beautiful National Parks and Federal Recreation Pass