He didn't spare time on his studies.
He grudges to give out information on his research.
He spends very little time on his studies.
He became less and less interested in his studies.
He has failed in the examination owing to his idleness―in consequence of his idleness.
He is second to none in diligence.
Hardworking as he was, sometimes he felt disinclined to study.
He is absorbed in his studies―engrossed in his studies.
He is wanting in ability, but he works hard.
He's as hardworking as any.
He does not work hard at the best of times.
He does not work hard at the best of times.