It is important to take him aside for this and not tell him in front of other people. So maybe if you find a time where you are alone with him in the room.
Then you want to tell him exactly what you told us:
Hey, do you have a moment to talk about something?
Non-confrontational, this way you don't start off on the wrong foot, and you will notice when you should better come back at another point in time. After all you want something from him.
Lately I noticed an unpleasant smell in the room [...]
Gauge his reaction. If he seems to want to say something, let him speak. Maybe it's not him and he has noticed it too. Then you wouldn't want to accuse him of something. Otherwise just continue speaking:
and I think it might be you, because of your football training. Could you please take a shower after coming home? Thanks.
This way you show him how you came to the conclusion that it is him and you also offer a solution. On top of that you don't directly accuse him of something and give him opportunity to explain himself.