Is baby overweight? Sometimes this has to do a lot with them sitting up, or even rolling over..? Anyway, my first child was very easy going at reaching his milestones.. but this did not mean that he wasn't loved, or interacted with... it was just that he was taking his time. He was slightly overweight for a baby, so this also played a big role in all of it. Doctors were aware, but since baby was happy. he was fine.
Personally, I would recomend that you make sure that your assumption of it all is not just an assumption... Have you held the baby, have you interacted with him well enough to assume that something is deffinately wrong. And by wrong, I mean, is he not interacting back with you, is he always crying, is he always sick, is he not rolling over..., if he not being cared for properly?
By the way, do you have any children? Are you experienced?
Because to tell you the truth, this is a matter of wheather you are a parent or not. It's hard when others that are not parents yet, make the assumption that because they are much older, they think they would know how to be a parent better. NOT! Love is what makes you a parent.. and these parents are loving their child, then this child is not being ignored. Now, if your concern is that this child is not being cared for properly, malnurished, etc., then you do need to talk to your fiance and perhaps have a talk with them about how you can help them, maybe even watching the baby so they can have a night out, and that way you can see how baby acts for more than the time you see him. You will see if baby is interacting, and babbing about with you... Some children are just serious untill they know you well..
I think a parent knows their child, and could probably tell if anything is not quite normal. It may take some time.. but maybe they are just giving baby time to develop on their own, and who is to say that they have already consulted this with their pediatritian. You don't know them well enough, correct? How would you know? This is not something that people run to tell someone.
If you want to keep things neutral at least for now that you aren't yet in the family, I would suggest you stay away from making comments unless you are certain that this child could not survive without your intervention. That is the bottom line. Keep yourself close to watch for baby's health. If he is always sick and crying, than that is a sign of malnurishment or that something is wrong. But if Baby is fine, then maybe he is just a quiet baby like mine was. NOthing wrong with that. Till this day, my son is a thinker.. but you should see the brains on him! :) I have 3 children, and all of them are quite different, and they all developed at different speeds... and offcourse the youngest is the fastest at all the milesstones that even I was amazed. I didn't know they could talk that much at 3 yrs old!! Sometimes that is just how this turns out.. they want to catch up to thier siblings. And yes, interaction plays a big role in that... weather it be with mother, father, or any other person... being able to express themselves is the key. Is he happy when you hold him, sing, etc. If there is no connection whatsoever, then perhaps there deffinately is a problem. If this is what is going on, then just simply make little comments while you are holding him.. saying.. "Oh my he is serious, etc. Is it me, or is he always like this?" And then maybe they'll tell you about it.. or they can perhaps start thinking about it. Then next time you see him say.. "can I hold him.. Let me see if he remember's me.. I want him to smile at me this time" And then if he doesn't react to you holding him say "I wish he would look at me... or smile?" Well, I dont know if this is what is going on, but at six months at least the interaction with another human is something that should be there as a sign of brain function, etc. And then reaching out, babbling about. Rolling over, and playing with hands... If baby is over weight this might be an issue still.. but if he is not, then there might be a problem.
There are so many little things that a doctor tells you to look for, so it all plays a big role. If he is doing something and not the other, then he might just be taking his time.. but if he is just sitting there with not reaction to anything at six months, then perhaps you do need to see if this child is being neglected. A call to Human Services.. anomymously would be something that can save this child if you need to do that. Only do this if child is in danger! Like if they leave him crying for hours, no one holds him, or changes his diapers, feeds him, etc. Those are noticable things, otherwise:
My advice would be either try to be closer to this couple and help them out with babysitting. Know the child good, and then once they are close enough with you, you can go ahead and make your final assumption... and perhaps have a talk with them how you all can help the child.
Good luck on your soon to be new Family!