In terms of propulsion, the Bosch crank drive system is reliable and gives a reasonably authentic cycling experience.
It's fitted to dozens of makes - and types - of bicycle, so it depends what father likes to ride.
As a general point, Cube/Bosch bikes look good value, starting at under £2,000.
Hub motor bikes are a bit cheaper, some of the Chinese stuff is truly dreadful, but there are decent makes.
Worth a look at Wisper - various styles available.
Still Chinese, but reasonable quality,
@welsh dragon has a Wisper which she is pleased with.
Wisper's owner, David Miall, is a good guy who believes in looking after people, so you will get some genuine customer service which you wouldn't get with a Chinese cheapie off
ebay.
https://wisperbikes.com/