Since your post doesn't give out more details, it is more a shot in the dark, but maybe one of them rings a bell with you:
1. Think about nutrition - I'm not going into the whole Jamie Oliver spiel but additives and processed food aren't great. If he has sugar spikes and troughs during the day, the troughs mean that he can't concentrate (and it's beyond his control, he won't be able to 'make' himself sit up and concentrate). Quick-release carbs predominantly have that effect, i.e. processed breakfast cereals, white bread, sugary drinks.
2. Is he getting enough sleep? My godson and goddaughter (twins) both suddenly needed 10 - 11 hours when they started 'proper' school and their mum didn't allow them to sleep that long - they were completely zonked out during the day as a result. When they came to stay with me for a few months (long story), you could really see the difference.
3. Tying in with 2., no stimulants such as coca cola in the evening, no stimulating activities such as computer, TV, etc. at LEAST an hour before bedtime. a) they will have trouble falling asleep and switching off and b) when they do eventually sleep, it is mostly rather disrupted sleep.
4. Finally, find out whether something else is on his mind that takes the focus away from school even when he is physically in the school building. Is there a family situation he finds worrying, has one of his pets died........anything like that can distract him from what he should do. As adults, we can compartmentalise (this is outside work - that is work - I'll deal with it later etc.), kids that age can't do that, or at least not to that extent.
Leaning towards cleaning. Am the Queen of my own castle.