When my son was a junior I rented a really nice f2.8 lense for the playoffs. I bought one for his senior year and sold it for almost what I paid for it once he was done playing. If shooting night games you will not be happy with anything less.
I found I could live with a shutter speed of 1/500 but that was the absolute slowest to get a decent picture. Not great but decent. Speeds closer to 1/1000 are preferred. If you use a lense 4-5.6 it's hard to get a decent shutter speed in night games even under good lights. It will also depend on what ISO you are happy with. My camera was decent at 6400 but much better at 3200 so that is what I used most of the time. I accepted a little blur for less grain. Everything is a trade off and I didn't have the money to invest in a pro body just to take pictures for football season.
One thing that worked well for me was to shoot more early in the season when you still have a little natural light. Late in the season gets dark well before kickoff and some stadiums are poorly lit for photos.
Plan to shoot from the sidelines and not the stands if you want great photos. The facial expressions you get up close are well worth the effort.
Also realize it will be hard to watch the game and take photos of your son at the same time. I watched him through the lense and paid little attention to the rest of the game.
