Difficult to make a suggestion without a budget, but there are any number of small SUV's which are smaller than the OP's Civic (which I personally wouldn't class as a small car), so an odd comment at the end there?.

My Dad, who's had both knees replaced (and having bother with them again) and osteoarthritis in his lower back, finds getting in and out of our Suzuki Ignis pretty easy. It is a tall car and quite a bit smaller than a Civic. He also found our previous car, a 2017 Honda Jazz, pretty easy though it isn't as tall as the Ignis.

Depending on budget (and assuming a manual gearbox), the VW T-Cross, Seat Arona and Skoda Kamiq (the youngest, so presumably the dearest) would be worth a look. They are the same cars under the skin and with the 1.0 TSI petrol engine would give a good balance of performance and economy (the biggest of the three is the Skoda at around 14cm shorter than the OP's Civic).

You could try something like a Renault Captur, it's quite a small car but sits a bit higher on the road. I'm sure others will recommend alternatives. You don't need a big SUV.

I was going to suggest that but having never owned a Renault was unsure.

When we had parents with mobility issues we bought a Nissan Note which under the skin was a Renault Captur of a previous generation. Not exciting but loads of space and access into front and rear was excellent as was boot space all in a car smaller than a Golf externally. Unfortunately this type of car has pretty much disappeared, the Captur is the last survivor. 2 cars prior to the Note we had a C-Max which was equally good for passengers but the boot was bigger, only downside was the height of the front passenger seat, mothers found getting in difficult and because of this we bought a conventional estate to replace it.

One simple piece of advice, take your father with you to view, you need to be sure the car is the right one.

Higher riding car/MPV/SUV is likely to be a solution.

However, as well as added height you have to be aware of other factors such as the distance from the door sill to the seat - wider space can make it trickier. Some cars have bolsters on the sides of the seat which again can be an issue.

A turntable can be bought, passenger reverses on to the seat and then, the turntable helps them get their legs into the car - some cars have wider doors which makes the leg swivel easier than others.

Nothing beats visiting garages and getting Grandpa to try getting in & out!

My parents had a Peugeot Partner Tepee as a Mobility car.

We have its cousin, a 2013 Berlingo, as our main car. It exactly illustrates the problem already mentioned of the gap between the seat and outer edge of the door cill.

My late Mother, after a bad fall, used a frame. Her technique for getting into a car was to shuffle back until her calves were against the cill. She'd then lower herself onto the seat and swivel round knees together in the way ladies of her generation used to avoid showing their stocking tops.

Picking her up from the hospital with the Berlingo she couldn't hit the seat. not even the edge, with her backside. Same both front and rear. She had no problem with the previous generation (2005) Berlingo.

Getting Mum in wasn't something we'd thought about when buying the car. If we had it wouldn't have changed our decision; I just use the other car when she needed to be transported.

Probably need to think carefully about how grandfather gets into the car and if necessary involve him in showroom visits or test drives.

Alternatively is there any way he can adapt his boarding technique so as to keep his head down getting into the Civic?