请教8 老师: F2A forward to NVC

First, thanks so much for your help and sorry for can't type Chinese on this PC.

We (me and my wife) are green card holder. Our 6-year son came to US with B2 visa in March 2015. In March 2016, we filed I-130 for him (at same time, we extended his B2 status to keep him on status).  In November 2016, his I-130 was approved (F2A Unmarried child under 21 of permanent resident).  But the I-130 approval notice says 

"The above petition has been approved. The petition indicates that the person for whom you are petitioning is in the United States and will apply for adjustment of status. The evidence indicates that he or she is not eligible to file an adjustment of status applications. This determination is based on the information submitted with the petition and any relating files.  If the person for whom you are petitioning believe that he or she is eligible for adjustment of status, then he or she should contact the local USCIS office for more information.

Because the person for whom you are petitioning is not eligible to adjust, we have sent the approved petition to the Department of State National Visa Center (NVC), 32 Rochester Avenue, Portsmouth, NH 03801-2909. NVC processes all approved immigrant visa which consular post is the appropriate consulate to complete visa processing. NVC will then forward the approved petition to that consulate.

This completes all USCIS action on this petition.  …..  The NVC will communicate with the person for whom you are petitioning concerning further immigrant visa processing steps”

On December 2016, we received a letter from NVC with a NVC case number. The NVC letter says:

 “The National Visa Center (NVC) received the immigrant visa petition filed by XXXX (me) on behalf of XXX (our son)  from U.S. Citizenship and Immigration Services. However, your case is not currently eligible for further processing because there is an annual limit on the number of immigrant visas that the Department of State can issue. For some visa categories, the wait could be many years.

This petition will be eligible for further processing when you priority date is earlier than the cut-off date for your visa category.

……………..

Do not make any firm plans such as selling property, giving up jobs, or making travel arrangement at this time. As soon as your case becomes eligible for further processing, we will contact you or your attorney (if you designated one). You do not need to contact us unless your mailing address or email address change, or there is a change in your life that may affect your right to an immigrant visa (for example: marriage, divorce, or birth of a child).”

My question:

1. Why our case was forwarded to NVC? Is this normal?
2. My son's PD will be current in next month, can we file I-485 (and I-765, I-131) for adjustment of status? Or we have to wait NVC to process our case (my so is in US now)
3. Do we need to go to local USCIS asking for filing I-485?
4. If we can still file I-485 in US, what we need to do with NVC?

Many thanks!