I have a back ground in Child and Adolescent psychiatric nursing as well as pediatric nursing.
I'm sorry to hear of your situation.
Thank goodness the child has you advocating for him.
For any child, it is important that the early years be filled with proper physical and mental stimulation since this is when the brain is forming. What occurs in these early years will effect the child for the rest of their life.
When there are delays, as there appears to be in the case of your grandchild, it is critical that evaluation be done as soon as possible, so that a plan can be put in place and resources obtained. If an intensive program is started, there is a good chance the child can overcome some of these delays (of course depending on the underlying cause). If a program is not started early, it becomes much harder to overcome the delays, as the brain begins to rapidly lose the excess of neuronal connections that it is born with. Un-used neurons are eliminated and the flexibility to learn basic language and other skills declines rapidly after the toddler years. The brains is like a 'sponge' n the capacity to learn in early childhood, but if the basic pathways are not put in place early on, then advances later in life are much more difficult (to put it in very simple terms). Of course, if basic emotional needs are not met at this age, an ability to form normal attachments will be damaged as well.
The child needs to see not a pediatrician, and will likely be referred to a pediatric neurologist and a developmental psychologist for evaluation, diagnosis, and treatment plan.
It is possible many of these delays are from lack of stimulation, if the child is neglected as you mention - and being set in front of a TV without without human interaction for hours is one form of neglect
I would think that you have a strong case to gain custody, based on what you have mentioned. And especially so, if the parents do not get on the ball and take parenting classes and chance their interaction.
Now, I don't see that you have a specific question, and you have asked for an opinion. I am not sure if I have provided you with the information that you need.
How else can I help you? Please do reply with any additional questions you may have, rather than rate me, unless you are happy with my answer.
I will be standing by to look for your reply.