My son has a swollen toe and foot after playing soccer today. How can I tell if it's serious?

Bruising & incr pain. The real concern is whether or not the toe has sustained a fracture. Signs and symptoms of a fracture would be increasing swelling, bruising (broken bones bleed), and pain or crepitance (crunching sound) would all suggest possibility of a broken bone. If pain does not improve overnight, an orthoapedic evaluation may be warranted. X-rays would definitively confirm absence or presence of a fracture.
Can he walk on it? Put ice on it tonight, if he is limping or not able to bear weight on it tomorrow i would get it looked at.