Hi J.,
This situation has me pretty stumped!
I'm not sure why your nephew is so resistant to eat solids. But the same hormones that maintain pregnancy CAN cause a lower milk supply. And at 10 months old, he should have already started on solids anyway-- not necessarly for his main source of nutrition- but just here and there, as snacks.
By now, most babies his age are snacking on Cheerios, etc. That he refuses everything is odd.
I actually wonder if maybe trying something sweet would help. I know that's probably like a sin to some people--
but if he's actually losing weight, who cares WHAT he eats, as long as he starts eating. Something like applesauce might be appealing enough that once it's on his tongue, he'll accept it. Hopefully, once he starts accepting ONE solid, he'll start accepting others.
Another possibility, if that doesn't work, is to force feed him. I have a disabled daughter who can't drink out of cup--she can't get the right lip closure-- so she only gets her fluids by sucking through a straw. But, when she's sick, she won't suck. So the only way I can get fluids in her, is to squirt them in her mouth with a medicine doser. I use a medicine syringe and just squirt it in her mouth.
With one of the larger syringes, your sister in law could do the same thing with a thinner consistency rice, oatmeal or barley cereal. You don't actually squirt it down they're throat-- you put the syringe over to the side, sort of over where their lower molars would be and just eject a little at a time. It's back far enough that the natural instinct to swallow occurs, but not far enough back that they gag. You have to put it a little further back in the mouth, because when you just put it at the front of the mouth, that's when they can just spit it out with their tongue. But when it's farther back, the natuaral instinct to swallow usually takes over. But again-- you put it back and to the side.
Hopefully, once he starts swallowing it, she can back the syringe up, closer to the front of his mouth, until she can even try it with a spoon.
But either way-- she definately needs to check with a pediatrician too.
Another possibility, is to check with a speech therapist about doing some oral stimulation. That might sound wierd-- but his problem could possibly be a physical one, and not just that he doesn't want to stop nursing and is refusing solids.
When we eat, our tongues move the food side to side in our mouths-- it's something that just develops on it's own. But especially in disabled children, that often doesn't happen.
I realize your nephew isn't disabled-- but there may be somekind of physical thing going on. A speech therapist, because he/she knows how the tongue and mouth work in eating, works with disabled children with feeding problems. She could watch your nephew as your sister in law tries to feed him, and may notice somekind of physical problem.
Keep us posted!
T.