How To Treat The Huge Bump On The Lower Shin?

My son was racing his dirt bike (motorcycle) Saturday and one of the other racers ran right into his shin full on with his motorcycle. That night he had a huge bump ( 5 by 5 in size) on his lower right shin. We iced it and I put arnica on it , it was extremely painful to touch.....he screamed every time I tried to rub it . The second day the bump went down and there was actually a divet in the middle of the effected area. Every day since it seems to be filling up with fluid and there is really no bruise? Which amazes me .....I am wondering if it could be a very deep tissue bruise r if anything could be broken? It still hurts to touch.