Leigh's mom and dad get divorced. As is often the case in such situations, the main reason behind their splitting up was money. Mom was always criticizing her husband for not having enough money to get the family out of their mobile home.
The problem was that Leigh's dad, who's a truck driver, wanted to own his own rig. But that cost a lot of money, money that his wife wanted to put towards a home of their own. In the end, the tensions generated by their disagreements proved too much to bear, and so they got a divorce.
As Leigh tells Mr. Henshaw in his letters, he misses his dad and wishes that he would come home with the family dog Bandit. It's evident from both the content and tone of Leigh's letters that he genuinely loves his father and wants him to be a part of his life.
At the same time, Leigh often gets frustrated when he doesn't hear from his dad for a while. Such frustration largely comes from the fact that he misses his dad and doesn't want them to become too distant.
In the letter he writes to Mr. Henshaw on January 15th , Leigh frankly states that his father isn't that much interested in him as he doesn't phone when he said he will.