I would recommend that boxset.
Also, probably the best western ever made:
The Wild Bunch
It all depends on his taste really, consider these:
Stagecoach (Wayne version)
My Darling Clementine
The Big Country
Pay Garrett and Billy The Kid
Rio Bravo/Eldorado (same story, different supporting cast)
Ulzana's Raid is one of my favourites, but try to get the uncut R1 version as r2 has alot of the horse scenes cut as BBFC deemed it as unnessary cruelty and it shows in the R" how badly cu t it is, anytime anyone fall s of a horse its cut.
it depends how old your dad is, or what period he prefers. my dad would like 50s/60s stuff better, the john wayne kinda stuff, like shane or true grit (which are both good movies), but i prefer the spaghetti westerns and clint eastwood stuf like...
good bad and ugly trilogy
a fistfull of dynamite
the wild bunch
once upon a time in the west
the magnificent seven
personally, if you like westerns you have to own those 7 movies. there are some good modern ones like unforgiven etc that others have mentioned. you can read up on the choices at imdb.com so you know what you are in for