24-105 is NOT enough for sports. 70-200 f/4 (either IS or non-IS) is the minimum equipment. For outdoor sports, the 70-200 f/1.8 non-IS is better than the f/4 IS
For outdoor HS sports, I use a 70-200 f/2.8 with 1.4 TC and find it is often not enough magnification.
have you ever been to a 4/5 year old soccer game?
he could probably walk half way on to the field if it's not enough reach