My husband has had a soft lump at the back of head on the right hand side. he says it's not sore to touch but is the size of walnut ?

Cyst or lipoma: Cysts & lipomas are often soft & painless. However, the former can become painful & even harden if distended & filled, much less infected. Lipomas, or fatty tissue, typically remain soft & painfree. Abscesses are collections of pus that are usually painful & need to be lanced/drained. Have him go see his family doc for evaluation & your reassurance.