Does my son have to wait for the approval of his I-130 before filing other forms? Do we need 2 G-325A forms in filing?

Yes, the priority date needs to be current in order to be able to file I-485. It is not enough to just have an approved I-130 petition. The priority date must also be current for the preference category under which the I-130 was filed.

That depends upon whether he is an immediate relative or has an available priority date.

Without knowing the immigration status of the person who will file the petition, as well as your son's current physical location, I am unable to provide an answer to some of your questions. But, assuming you are a US citizen and your son if under 21 years and physically present in the US after lawful admission as a nonimmigrant (such as a B-2 visa) even if it has expired, you can file the I-130 and I-485 applications with I-765 work permit at the same time. Also, both you ( the petitioner) and your son ( the beneficiary) must complete your respective G-325A biographic forms.Because, it does not appear that you are familiar with this process, and to avoid making potentially costly mistakes, you should speak to an immigration attorney.

It depends on the facts of your case. If the priority date is current.

The answer to your question depends on your status and his age. If you are a USC and he is your son and under 21, he can file everything together. If not, then you file the I-130. He is put into the quota system and at some point will be eligible to file the I-485. I am NOT saying that he is eligible to file anything; for that answer, I highly encourage you to set a consult with a competent immigration attorney. In fact, I feel it is very important that you do so, in order to fully evaluate your son's case.