My 7 yrs son has headaches about once a month. he goes to school from 8 am to 4 pm and twice a week does sports after school, his headaches get terrible after that when he's home. but after a night of sleep he gets better the next day. is it normal?

Not normal but not something you have to worry a lot because the headache occurs randomly only one a month. You should try to make a diary on the date he has the headache, such as school works or what sport activity involved on that specific day, any issue with eating, hungry, thirsty, etc. From then you can figure out what the trigger is or you can present those findings with a pediatrician.