His grandchildren will only be able to visit a few weeks out of that 6 months, so he should choose based on what HE wants to do with his days. If he is into fishing, for instance, it makes no sense at all to live that far inland. Proximity to medical facilities is very important (will he use the VA health care system?), as is proximity of services if he has to give up driving -- you don't want him to be 15 miles from a supermarket. I recommend NOT trying to combine trips to see Dad with trips to WDW unless you sequence them; it's really never easy to venture too far away from LBV in the middle of a Disney trip.
My DS lives in Pinellas county, and there are a very large number of over-55 complexes there that cater to snowbirds, both in the cities and on the barrier islands. (I know this all too well because the most affordable places won't rent to younger residents, so he had a hard time finding a place he could afford.) Pinellas, in particular, has a very large number of 55+ complexes that keep costs quite low in the rental market, though downtown St. Pete is no longer the retirees' haven it once was -- downtown has gone hipster, and is very much the territory of the young these days.
I'd definitely recommend that your Dad try renting for a season before purchasing a home, wherever he decides to go. (Many older folks who are not into golf find that they really like beach-walking for exercise. That works if you are under 30 minutes from the shore, but not so much when you have to drive 90 minutes to get there.)
If he wishes to establish Florida residency for tax purposes, be aware that the requirements for that are strict and require establishment of multiple commercial relationships with Florida service providers, such as bank accounts, insurance policies and medical care arrangements. Consulting a Florida attorney about the process is highly recommended.