Legal Question in Family Law in Texas

First, you need to consult with a CA attorney on this issue if he has lived with her 6 months or more.

Next, Hypothetically based only on what you provided:

If your son has lived with his grandmother for 6 months or more in CA, the suit to establish custody rights may need to be filed in CA, if there are no other previous orders entered in another state - and if she chooses to fight you to keep him or get him back. If there are other orders entered in a different state, the original orders from that suit may need to be transferred to CA or possibly the suit may need to be brought in the original jurisdiction and venue where the original orders were entered, but, Research will be required in order to determine these issues.

Based on the laws in Texas and Arizona, IF your son lived with his grandmother for 6-months or more - she has now established a substantial relationship with your son and if she chooses to fight for custody she can, and there are a lot of things that she can do to you, after going to court, including possibly getting a Writ issued against you by a Court Judge to return the child to her if you take him, and, if that is done, the Sheriff will take him from you, and that Writ can include orders from the court removing your custody rights to your son. You will be able to file an Answer and counter her suit of course. And, if she files a custody suit against you, she can also ask the court to make you pay child support and attorneys fees. She can also ask the AAG to file an action against you to get child support from you and his father, if the child has been living with her continuously, and that may precipitate a court order including custody, which she already may have done?

Next as to the Amber Alert, I would consult with the CA police dept. on that question or the CA attorney.

You may want to try counseling first, to see if you can re-establish a relationship with both of them.