So I have a player that wants to play a werewolf, and I'm not sure if he can or can't. He's very interested in it, so is it possible for him to do this? And if so how does character creation work?
It's up to you as the DM. From a purely rules-based perspective, absolutely he can. From a storytelling perspective you will need to consider a few things.
The rules do have support for this, and if you have the 5e Monster Manual, you can read the rules for werewolf players in the sidebar under the Lycanthropes section (where all the "were-" monsters are listed). You will need to consider for yourself the type of game you want to run and the powers you want your player character to have. This has the potential to offset the balance of standard character progression, which could potentially affect your CR calculations when determining appropriate encounters for the party.
You also should consider the effect this might have on other players at the table. It could create spotlighting issues or other table-balance problems. Werewolf characters tend to have built-in story hooks and personally I'd be concerned with that character derailing any plot lines that don't involve him directly. Your mileage may vary and of course I don't know you, your group, or your player, but I would be hesitant to allow this as part of a character concept.
Character creation is unchanged, whether he plays as a werewolf (or other lycanthrope) or not. As lycanthropy is treated as a disease/curse, it is applied the same way to any character of any level. He should make his character as normal (without considering the lycanthropy curse) and then apply any modifications that come from lycanthropy after character generation has been completed.
@LegendaryDude's answer does a great job of addressing the 'can he' part of your question so I'll give another option for the 'how to' part of your question.
The Eberron Unearth Arcana free download contains the race of Shifter. They are described as being descended from humans and lycanthropes. They can only turn into a hybrid form (think classic movie werewolves of a human in a furry suit) as opposed to the full D&D werewolf which has a hybrid form and a wolf form.
Compared to other standard D&D races a Shifter would be more balanced that a full werewolf (especially at low levels) as it doesn't have any damage immunity like a werewolf. It does have nice abilities tied to its hybrid form and these abilities are less powerful than the werewolf's shape change which can happen as often as desired.
It would also give the DM more flexibility to work into the story as Shifters lack a lot of the negative storytelling aspects that a regular werewolf could bring to the story. A Shifter is just another minor race with its own history and prejudices (or you can make your own to fit into your world/story).
A bit late, but if other sources are OK, there are classes. If you get the Touch of Class PDF there's something called the Occultist which allows you to play an abomination, a vampire, or a werewolf as your class, so you would keep control and it'd be even. In fact, looking over the class, they start off in a worse spot than Barbarians: Action to shift (though it is permanent until dismissed by an action) Don't get resist until level 7 (and its easier to bypass) Don't get Barbarian AC rules until 10 However once they get going things get better: At level 10 one of their resistances becomes an immunity, gaining another at 14 and 18 They get unarmed dice as a "Dark Strike" and for a bonus action can add Con and does Necrotic damage for their attacks, and the dice are a bit higher than monk, they go up to d12, but only get 2 attacks. Additionally, all Ocultists get 3 of these 4 as they level: Speed Boost, Climb Speed, Fly Speed, Burrow Speed So a Were-Occultist is like a combination of a Monk and a Barbarian.
I reccomend the PDF, it has some cool classes, even a Yu-Gi-Oh reference.