I posted earlier on my relationship breaking up this basically comes from there,

My ex is very close to her parents and to be blunt me and her mother despise each other I spend a lot of my time away in the military sometimes up too 4months and my ex is always wth her parents they see my son almost everyday and he stays at least once a week I am terrified he will start seeing them as more his parental figures than me my ex has already commented on how much of a better bond my son has with her dad than with me I spend all the time I can with him and give him everything I can, he is only 2yrs old and obviously he is learning that his granparents are around more than me, Is my only option too come out of the military? Unfortunately though I have a pretty bad criminal record for fighting a lot when I was younger so I would struggle to get a decent income and keep my son in a good life whereas my ex's parents are financially worlds apart from me...

Every chance you can assure him of your love for him. When you spend time with him make it very special for him so he grows up with great memories with you and wants to be with you when he is old enough that they have no control over him. You should be able to go file for joint custody at some point. I don't know if it would be worth it if you can't spend much time with him since you are in the military.
I would think that your being in the military would over ride your problems when you were younger to a potential employer. Before you decide whether to leave the military have a good idea of what you would want to do and where you would apply. Have a goal.