Our muscles are not damaged, it’s the nerves, our message senders, that are damaged and they cannot tell the muscles what to do. I cannot sit with my legs crossed because it feels like bone pressing against bone, so I know what you mean.
Try a “SOLID FOAM” pillow (not shredded) for your husband to sit on. I find this more comfortable then gel or cotton batting. I am in a wheelchair and do alot of sitting, good thick foam works best for me.
Your husband may need a maintenance series of IVIG’s rather then just an aggressive series over five days. I was on IVIG one day every two weeks for about 2 1/2yrs.