As the resident physical therapist in training I'll take a stab at this one...
The best treatment for any overuse injury in the body generally begins with rest and activity modification. If activity modification isn't possible, then sometimes orthotics or shoe changes can reduce the loading on the involved structures. Medical treatments could involve the use of pain killers or corticosteroid injections to maintain function, but both serve to mask the symptoms, and neither provide tissue repair to solve the problem long term. Furthermore, repeated corticosteroid injections have been linked with decreased tissue strength and a greater risk for injury in the future.
The greatest torque on the plantar fascia occurs when the toes, especially the great toe extend. In the gait cycle this occurs during the final phase of single limb support, directly before the foot leaves the ground to advance during swing phase. Stiff shoes could limit this toe extension, and therefore decrease symptoms, but there is a trade off also with shock absorption.
The night splints are an excellent idea to prevent toe extension during sleep and allow the plantar fascia to heal more. Furthermore, any modalities to reduce inflammation, stretch the plantar fascia, and improve the strength of his intrinsic foot muscles will improve his condition. Since the plantar fasciitis is mild it should resolve with these interventions. I would estimate that he has a very favorable prognosis for recovery of full function well before the end of the season if he is consistent with his treatments and finds a pair of shoes to reduce the stretch of his plantar fascia.