My son , who is a green card holder (not US citizen) went back to the Philippines to continue his studies in college. How long can he stay in the Phil

Any prolonged absence will indicate your son has abandoned his intent to remain in the US permanently.  He should plan to return to the US regularly while studying in the Philippines and he should maintain strong ties with the US during the entire time he is away.   His absence will affect his ability to apply for citizenship, which requires a continual presence in the US with only occasional absences.