Ideally, he would need someone to sit down and study with him. Encouraging him and refocusing him when he goes off track, but always in a positive mood. That takes a lot of time and effort, though, so I don't know how feasible it is for you. You may want to look up the concept of scaffolding in the context of ADHD for this.
Things to try for him are listening to music while studying (some of us perform better with added stimuli) and having a source of glucose ready (glass of orange juice or similar to sip on). Some research I've read suggests that ADHD brains consume inordinate amounts of blood sugar when focusing.
Medication such as methylphenidate can also help but should only be administered following guidance from his doctor.