NanaDiana my DD lost a lot of body weight quite quickly when she had bronchiolitis as a baby but regained it very quickly when she started feeding again.
I assume he is being breastfed which is the best defence against catching anything else. He is too young for solids so all your DIL can do is feed on demand and stay relaxed so she has plenty of milk for him.
Babies usually (not invariably) sleep and travel well at that age.
They could register as visitors at a surgery when they arrive here.
Good Morning Good Friday 29th March 2024