Are you entering into the military?
I am not.  I am actually asking for my son who will be leaving for basic training next month.
if my son's license is valid now, what will happen if it expires while he is in the military?
If his NY State license was valid at any time during his military service, his license will be able to be renewed.
What happens if my son has a lapse of insurance coverage during his period of active duty?
He is required to maintain continuous coverage or to surrender his plates before any deployments.
Is he unable to ensure that he will maintain continuous coverage or to surrender his plates now, before he departs for basic training?
That is correct.  He is unable to pay forward enough on his policy to ensure it would give him continuous coverage for the period he will be gone and he is unable to get to the DMV during business hours to surrender the plates at this time.
Basic training is not a deployment.  The DMV does have a procedure in place to exempt military personnel from the requirements, however the procedure pertains to returning from deployment.
What would happen if his NY State registration would expire while he is on active duty?
Registrations are extended for up to 60 days from return to NY State.  This extension applies only to vehicles registered to military personnel.  He would need to show proof of military service or separation.
Is there another resource I could use to get more information?