When you say you talk in the evenings, are you actually physically together or are you talking over skype, something like that? If it's the latter, then turn off your phone/laptop if you have to, or switch off the internet while you're trying to work, after you've told him you'll be revising for X amount of time, and you'll speak to him at Y o'clock. That way you aren't cutting him off completely, and telling him he will get to speak to you, but that you need that other time to revise.
If you're physically together, then either tell him not to come over, or again, explain that you'll be working for X number of hours in a specific place, so the door will be closed and he's not to interrupt. As others have said, suggest things he can do: watch TV/a film, go on the internet, play games. Then when you're done for the evening, you can join him & have some time together without feeling guilty.
All of these scenarios rely on you being firm but kind with him. Explain how important the exams are, that it's just two weeks of work, and once you're done you'll be much freer to spend time with him. If he's a decent guy then he'll understand, even if it's hard for him since he's mega-clingy. But if he just can't leave you alone, then consider other options: study at school or the library or even a coffee shop until a certain time in the evening, and only then come home.