I use Public Mobile, and have been quite happy with them. For your dad, something like the $25 a month plan might work well. For making calls to the US, you can buy add-on packages for certain amounts of minutes, or you can use SunnyDays' suggestion of internet calling. Google Hangouts also offers free calls to the US over internet, and is what I use whenever I'm making a call to the US.
One other benefit I've found with Public Mobile is that they have free unlimited incoming calls, which also includes calls from the US. I'm on the $15 a month plan, and don't pay a dime or use any minutes for any US calls that are placed to me.