4 Month Old Breastfed Grandson Dropping in Percentile Rapidly, Not Having Any Bm

OMG-PLEASE understand that a breastfed baby's digestive system is so completely different than a formula baby. That is first and foremost. There is plenty of clinical evidence that a breastfed baby will often go a week without a BM, and this is COMPLETELY normal. All my kids did it. In fact, it was wonderful when they got to that point (all around ther 3-4 month mark), b/c it meant less diapers! All my Exclusively breast babies were CHUBBY little ones! So, I never did experience one that wasn't gaining on my milk. When a baby is exclusively breastfed, there is often times where the milk is so perfectly proportioned for that baby's body, that there simply is no waste products to come out as a BM. On formula, there is so much of that that a baby isn't made to digest (after all, it is milk made from cow's milk, made for baby cows, not baby humans) that a BM is how they get rid of undigestable and/or unprocessable parts. Same as adults and kids on a regular diet.
Now as far as not gaining, my hunch would be that this is a caloric intake problem for mom. At pregnant and breastfeeding, she should be counting calories to be sure she is getting AT LEAST 2800 each day, to support both the fetus and the baby. Make sure she knows that.

ok, 1) it is normal to not have bms in a breastfed baby for 7-10 days 2) it's also normal for them to gain very quickly the first 2-4 months and then fall off the charts steeply as the US charts are not designed for breastfed babies...My supply when pregnant didn't drop until I was 5 months pregnant, so in a healthy relationship it's totally ok and very easy to nurse while pregnant with no issues at all. Do you have a LLL, or a certified lactation consultant you could see--they would have a scale accurate enough to see how much he's eating.
Did he gain weight at all, even a few ounces? Not gaining at all could indicate a supply issue with her being pregnant, though it's not common until they are fully into the second trimester for the supply to drastically drop...some babies are meant to be small...my first started at 5% on the chart and was 90% at 6months, she's still there at 10 years old, my middle child started at 10%, got to 75% by 2 months, 50% at 4months, 15% at 6months and at 2.5 she still hovers between 5-15% for weight...her height has always been 75%
Is he growing in height? head circumference?

If nothing came back abnormal, I would wonder if he is jaundice...you know yellowing? It seems that the doctor would be able to see that though.

I would try and put him on formula and see if that makes a difference. It seems that the docs would list him as failing to thrive. Both of my kids had problems and had to go on the Alimentum formula because of constipation. It seems to clean them out pretty well. Sometimes the doctors don't think to try it because it is isn't on their "radar" but it can really help.

What is Mommy eating, and how much is baby eating, especially now that she is pregnant?? If he is too backed up, he won't want to eat, and that keeps him on a cycle because then nothing goes in to stimulate the natural flow. He might be too young for a laxative, but is there anything that the doctor can suggest along those lines.

The main concern that jumped out at me is that I hope that this current pregnancy doesn't take attention away from him in this critical time and lead to any failure-to-thrive issues. Oh, they are so delicate at that age; so much uncertainty.