Are you talking about the night shift? I guess probably not because if they saw him once, they would have been sleeping the rest of the night.
If they both work, come home and ignore their child, and you observe that day after day, it would be a huge cause for concern. There is a possibility that they haven't bonded well with their child. You don't mention his age, but it's very common that disability and frequent hospitalizations over a protracted amount of time are very disruptive to normal parent-child bonding. This is associated with some negative outcomes for both.
Another possibility is that while they are great parents, the relentless day in day out care for a child with those needs wears away at families and causes psychological weariness. In that context, a desire to escape for a little while doesn't mean they are bad parents. It means they might be feeling depressed and hopeless. Knowing that your child will probably never get better is very daunting.
I wonder what you observed when the parents did come in to see the child. If they patted him on the head and left, it's one thing. If they talk, pick up, touch and kiss him it's another.
It's good he has you to fill in some of his need for interaction, but if the parents are truly detached they might need some sort of counseling- best wishes!