Should I be concerned?

Bella had kittens last week. She had three and two died the last one seems to be surviving. He's nice and pink and warm she rarely leaves him an he nurses. What had me concerns is that when she does leave or I pick him up his meow is more like a tiny croak and very low. Like I can barely hear it. But his breathing rate is fine. In all aspects he seems to be perfectly healthy but his meow used to be very loud. Is there something wrong with him? Or am I just over reacting?

I would take them both to see the vet.  There are  lots of members with more experience than I have.  My opinion is that it is best to ere on the side of caution.

You mean, he meowed more loudly earlier, but is now meowing very faintly?

Otherwise, some kittens learn to meow firstly at several months age.  They probably do have some sound which cats hear, but not humans, as usually one can see they are moving their speaking apparatus, opening up the mouth.

If you do have access to a good vet experienced with cats and kittens, I would second CCCDL´s advice.

Good luck!

It's not that he doesn't meow or that he can't he just sound hoarse almost. But other than that he seems perfectly healthy. I would take them to the vet but I can't right now because I just had to pay for a c section for Bella and have no more money.

 There is a lot of good advice to be found here.  Make sure the baby is nursing and eliminating.  Check temperature and weight.  Feed momma LOTS of good stuff.  Keep the baby warm.  (give it access to a covered heating pad) I know what it's like to live paycheck to paycheck.  Juggling all the bills and then the unexpected. Just keep watch.  One thing I think is good for tiny ones... momma kitty purring.

He is being kept plenty warm enough by Bella. I did call the vet and he said just to watch him but like i said there doesn't appear to be anything wrong with him except he doesn't meow loud