Tell him that if he is at home, I’ll call to see him.

that引导定语从句要注意以下几点

1.定语从句中缺主语,修饰人时用who/that,修饰物时用which/that引导。如: Is this the factory which/that makes TV sets?The man who/that visited our school is from Australia.
2.从句中缺宾语,修饰人时用who/whom/that或省略引导词;修饰物时用which/that或省略引导词。如:Gone are the days which/that we spent together in the village.He is the man who/whom/that I think to be worthy of our praise.