(Shrugs). There's a lot of Republicans who don't like Ron Paul because of his Anti-War stands.
Rand Paul is a little more hawkish than his dad on some of these subjects, such as his support for the War in Afghanistan and his support for military tribunals at Gitmo.
Those are probably the two biggest Foreign Policy differences between the two.
Daddy Paul is Anti-War because he understands (rightly) wars are economic disasters. What he can't get through his thick head is that most of us don't favor killing Islamic lunatics for economic gain (or fun), but because killing them is the only alternative they will permit other than them killing us.