The XSi tops out at ISO 1600 which means it is only going to be useful for games that are in daylight so depending upon when your son plays, your camera may not produce usable images unless you have excellent lighting. That issue is made worse by needing to crop if you aren't close enough and having lenses that don't let in a lot of light (lenses that work well in low light get expensive quickly).
If his games are primarily during the day, then the XSi is worth trying but if the games are played at night then I would be looking for deals on other used bodies. That camera design is around 16 years old now and sensors have improved markedly over that time period with huge gains made in the 2015-2020 era.
I starting coaching youth soccer shortly before your camera model was produced and I was using a Canon 1D Mark II which was their top sports camera of the time. It had a maximum ISO of 3,200 and even using an expensive (around $2,000) lens the images once the sun went down were pretty bad.
You don't need brand new but I would look for a deal on a much more recent used camera. I hate phone cameras but a lot of them will now have better performance than a camera from 2008.
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
