My son is 17 and has started his two a days for football . this am before practice he weighed 261 after practice 251 is this ok?

Mild dehydration: The body is roughly 70% water & shifts of this amount 10lb in 260lb person amounts to less than 5% of his body weight. Although troubling, this is something he should be able to tolerate. Fluid loading prior to workouts & drinking more during breaks could avoid the weight loss. Failure to keep up with his fluids can lead to dizziness, declining performance & heat related illness.