He always finishes doing his homework----time

on time .
on time有“准时,按时,用分期付款的方式”等意思.如：
The trains run on time．火车很准时.
John bought a car on time．约翰用分期付款的方式买了一辆车.
而in time有“来得及,不久,总有一天,合拍子”的意思.如：
She will be back in time to prepare dinner．她会来得及回来准备晚餐的.
I will see him in time．总有一天我会遇见他.