are you sure the spare fits your dads aswell???
It may well be a series steel rim in which case it might be tight/or mega mega close to fit over the hub centre. Most have been "touched" with a grinder by now.Or laying on your back stomping them on has worked in the past to get a motor mobile off a safari course. Make sure the wheel you have fits your old mans tho.
There could be soooo many reasons one could be that your old mans has later axles fitted and alloys which usually fit the rears but not the fronts on older deep type drive members. I'll stop guessing now....
Is it a steel wheel?? Pic??