As per my understanding, if you are not with the same employer who originally filed PERM and I-140, you have to do a new PERM and I-140. I am sorry that you have to go through the kid age-out scenario.
The age-out stops only when the "FA" dates are current. Right now for your PD, only the "DF" is current. So your son still faces an age-out scenario when he is 21. I would suggest you to count 180 days, use AC21 and port to a new employer and file PERM/I-140 EB3 application immediately. PERM / I-140 if you speed through can be completed in one year time. The "FA" for your PD is most likely to be current in EB3 before EB2.
I know people are very optimistic about big movement of EB2 this year (including myself). But until that happens nothing is certain. You have a pressing deadline / age-out scenario. So take whatever steps you can to lock your son's age. So IMO you need to have applications in both EB2 & EB3. The waiting game from 2012 has given me the experience of doing whatever we can open multiple lanes for path to GC. If EB2 FA becomes current that will be great. However, we need to also prepare for the worst case. Good luck.
Edit: Forgot to mention that prepare for AC21. Get copies of approved PERM so that you can have a)SOC code, b) Job title and c)Job description. If employer is not sharing PERM, use FOIA to get older copies.