The analyses of a text #2

The protagonist, Philip, who was born with a club foot, moved in with his uncle Mr. Carey, the Vicar of Blackstable after his mother's death.
He came to London to work as an apprentice clerk for the accountant Mr. Carter for five years.
The other apprentice clerk Watson, a son of a brewer talks to him.

...........................
"You've got five years of it, haven't you?" he said, waving his arm round the tiny room.
"I suppose so," said Philip.
"I daresay I shall see something of you. Carter does our accounts, you know."
Philip was somewhat overpowered by the young gentleman's condescension. At Blackstable they had always looked upon brewing with civil contempt, the Vicar made little jokes about the beerage, and it was a surprising experience for Philip to discover that Watson was such an important and magnificent fellow. He had been to Winchester and to Oxford, and his conversation impressed the fact upon one with frequency. When he discovered the details of Philip's education his manner became more patronising still.
"Of course, if one doesn't go to a public school those sort of schools are the next best thing, aren't they?"
Philip asked about the other men in the office.
"Oh, I don't bother about them much, you ," said Watson.
"Carter's not a bad sort. We have him to dine now and then. All the rest are awful bounders."
[Of Human Bondage by Somerset Maugham]
1. I'd like to know what "Cater does our accounts" means.
2. I'd like to know whom "one" refers to.
3. And I'd like to know why it is "have him to dine," not "have him dine."
Thank you in advance for your help.