I want to ask my boyfriend to take his grades more seriously
What is hindering his grades?
He's spending too much time on the football team
You are a good girlfriend for being concerned about his grades for him, because football isn't everything while time management is. If he can't be serious about his studies, he can't rely on football his entire life to carry him.
I'm a thoughtful person and I want what is best.
It is good to be supportive of him. Help him to work on a way that allows him to still play football while keeping his grades up.