My husband wants to try adult nursing is that healthy? And how can I start lactating again?

A little is ok. Breast milk is very nutritious, and excellent for babies, so will be of no harm to adults (in small amounts). Breast milk is too rich, so will give an adult too much animal fat (yes, we are animals) and many extra calories (not good for cholesterol levels and one's arteries). A woman may have a hard time re-starting lactation if she had stopped weeks or months ago.