What he is doing is shirk while taking medicine is advised in Islam!
A silver knife is like a sacrifice presented to the Jinn, asking them to leave the child and this is shirk.

My husband placed a silver knife by our baby’s pillow to protect him from jinns that upon seeing such materials would run away. We had a big fight about it when I told him to make dua upon the baby and that will protect him In Shaa Allah than a knife. He told me why do I rely on medicines although I know Allah can cure everything? Please help me, what do I tell him? Is this practice permissible?