Shouldn´t it be
Billy had more money than he did
In Gmatland, I´ve only run into "I had", "you had", etc, when had was used as an auxiliary to form the past perfect. For example
I had left two hours before the club closed, and they had too.
Also, whether the subject or the object form of the noun goes after than depends on the sentence.
1) If you say
She has more money than he does
She has money, he has money. He is the subject of the 2nd clause and thus keeps the original subject form of the noun.
2) If you say
She gave more pencils to Billy than to him
She gave pencils to Billy, she gave pencils to him. Him is the object of the 2nd clause and thus has to be in the object form of the noun.
Comments/suggestions welcome.