My 17 Year Old Son Was Playing Football Last Night

my 17 year old son was playing football last night and got hit in the knee on the side of it as he tried to jump over a player.
when he went back to the line to squat he got nauseous and said it hurt to bend it.
the trainer said liagaments seem to be intake, but my son says it hurts to bend it and hurts even more to straighten it after bending,
there is not swelling