My old ADV was a bit complicated, but basically it was cold steeped tobacco in VG, with a hint of tobacco absolute, menthol, koolada, a couple drops of a good citrus flavor and just a touch of your favorite mint. Its a pretty glorious mix that can taste exactly like a freshly opened pack of cigarettes smells.

10% ry4 (tfa) is good by itself.

My ADV for years was nothing but this:

70/30

DK Tobacco 5%

Most poeple use DK Tobacco just as a base, but I still love it just on its own. By itself most poeple seem to either love or hate it. Its a fickle mistress so you will need it steep for at least 1 to 2 weeks... you could even even shoot for three because unlike some recipes, vaping it before hand it doesn't really taste anything like it does fully steeped.

Otherwise I saw another DK Tobacco recipe posted on the e-liquid calculator by someone named ted

And its pretty much the same as above other than just adding 1% TFA M-Type:

I tried it and liked it but I thought it took off a bit of the sweetness of the straight DK Tobacco flavor so I just went back to just the one flavor.

And your father may begin liking other flavors after a period of time, at first for me just coming off cigarettes I wanted it to mimic nothing more than just a cigarette. I didn't necessarily dislike other flavors, but tobacco flavor was the only thing I wanted to vape for a long time, its what I was most comfortable with, it kind of mimicked a cigarette. Now I pretty much like everything, but at times I'll go back to this recipe for my ADV until I feel like something else. It just took me awhile to enjoy other flavors.

Mix up some TFA Western (Cowboy) @ 5% for a straight tobaccoesque vape. 80% pg if that would work for him and give it at least a 2 week steep. I use it as a base and if he likes it you could tweak it down the road.

I've got a simple 3 flavor recipe. Desert Ship Tobacco 15% Honey 5% Pecan 5% I named it Sweet Honey Bacco

I really like:

 7% FW Coumarin Pipe Tobacco
 1% FW Butterscotch
 1% TFA Marshmallow

Nice and simple. Tastes pretty good right after mixing, gets better with time. Best of all, it's almost clear at 6mg, and doesn't gunk up my coils like many tobaccos.

edit: Just re-read your post and saw he doesn't like flavored tobacco. You can replace the butterscotch with a tiny bit of tobacco absolute for a stronger tobacco note if you want, although you don't really notice the butterscotch to begin with unless you really look for it.