假设Rober = 张小三，his father = 张三。

原文Robert wanted his father to know how much to love his father.

=张小三想让张三知道（张三应）如何爱他的（张三的）父亲。

改文：Robert wanted his father to know how much he loved his father.

=张小三想让张三知道自己（张小三）有多爱他（张三）。

结论:

原文本身没什么问题：儿子要老子知道如何去爱老子的老子（儿子的爷爷），根据提供的改文来看，原文属于意思表述失误--儿子不是要老子知道如何去爱老子的老子，而是想让他老子知道自己有多么爱他。哈哈

原句是错误句，所以你也不必改了。

名词从句简化为wh+to do时，原从句还有情态意义，所以还原后是

Robert wanted his father to know how much he could/should/would love his father.这句话不知道要表达什么