The tropicals are interesting balls...but there isn't a whole lot of core in them so your father-in-law could experience some bad carry with it if it doesn't match up to his style and prefered shot on the lanes. I would personally look at picking up a Dynothane Crisis Pearl as the ball can handle a little bit more oil and it has a better core to help with hitting power.
After that it depends on how much of a jump up he want to make in ball reaction, where he likes to play on the lane, revs, etc.
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-Chris: DJ's Pro Shop : Auburn, MA