If you have alot of time and a good camera, you can take the pictures yourself. Go to the school, before school starts and photograph that grandma is dropping him off and picking him up, or if he rides the bus, take pictures of him getting on and off the bus at her house. Also take the pictures to a place that will date stamp the negatives and pictures for you as verification of the dates.
Also, you can take photos of your ex going to the "girlfriend's house and the fact that he never leaves once he gets home from work. You can video the entire night that his truck car whatever is there from time off until he goes back to work again. Do this for several days to show that he does not return to his parents house to sleep ever. Again be sure that the camcorder you use automatically date stamps the film. Also you can go to a store that sells the cameras and go to their service dept and have their personnel set the time and date on the camera and have them give you a receipt showing that they properly adjusted the camera to the exact date and time. This will help to ensure that you are not accused of manufacturing a date and time on the camera.
For further safety with the camera. Go back to the same store and have them check the camera about once amonth with documentation that they are checking the date and time stamp so you don't look like you have changed or manufactured dates on the camcorder. Keep the receipts in a file together so that you can present this to the judge in the event he trys to say that this is the same tape with the dates changed.
Also, if you are sharp enough, you can start by asking questions of the school. And get statements from them that they are dealing with the grandparents.
Also, if you can, try to track down what address the g-parents are giving the school as the child's address and see if you can get the address where your ex is living and show that the addresses are different.
Don't mess with the mail at either place though because that's a federal offense. Also this is gross, but try to sneak through his trash occasionally to see if you can prove he is receiving mail at his girlfriend's address.
There are all sorts of ways that you can get the info you need without seeking the help of a PI or an attorney. You just need to be sneaky enough.
There are a ton of ways you can get the info you need. I would tape all conversations with the grandparents, the child, etc on the phone. I would tape conversations on the phone with your ex.
Another way is for you to tape conversations where you call your ex- and ask to speak to the child. Do this on at least a weekly basis on different days at different times. Record his response that the child is at the g-parents. If you do it long enough you may actually agitate him into telling you that the kid is living with his parents and to stop calling his house and call the grandparents.
Since Texas is a one party recording state you can legally record any conversation you have with anyone without informing them that they are being taped.
After you call the dad and he tells you the boy is at grandma's. Call grandma's house and verify that the child is there by talking to him. I would do this several times. Sometimes calling late in the evening right before bed time and ask him if daddy is coming to get him to take him home to go to bed and then you are in the position of recording the child saying, "No, I sleep at grandma's all the time." After he says that, ask him when daddy comes home. Then you have the child taped saying, daddy doesn't live here. He just comes to visit sometimes.
There are ways, you just have to be really devious.
I"m probably going to take a lot of heat for my suggestions on this one, but oh well. People are going to accuse me of using the child as a pawn, and unfortunately what I am suggesting would be using him has a pawn, but it will provide you with the info you need to pursue the custody issue by showing the father "who is to have primary custody" as having abandoned his custody "by willfull action of leaving the child in a residence under the constant care of another."
BTW do you pay child support? If so, is it going to the dad or the grandparents? If it's going to the dad, you can't be sure it is providing for the child. You might be able to ask grandparents if he is giving them the support money once they are aware you have uncovered the fact that the boy is residing with them and not his dad.
You can do it but this will be extremely time consuming and will be difficult, but if you have the time to kill and you want to try to force him to give you the boy it may work.
However be prepared for it to end up that he either moves back in with his parents, or he takes the boy with him to live with the girlfriend. Either of these two things could also happen. Which would mean you spent alot of time manipulating all of this only to still not have primary custody of your son.
Also if you decide to become your own PI, do everything you can to present yourself as having emotions of steel. Don't get angry, don't be demanding, just do everything you can to show that your ex has abandoned the care and custody of his child to his parents.
I'm sure I'm in for alot of hate mail on this one from ex's who can't believe that someone would be this intent on taking a child.