I have family shared plan which has been shared with my husband and it keeps asking him to join apple music. what do i do?

Just to be clear, did you setup Family Sharing according to:

Set up Family Sharing - Apple Support

and invite him to join? And did he accept that invitation?

Then, did you get an Apple Music Family subscription according to:

Get an Apple Music family subscription - Apple Support

If so, you might be able to fix your issue by turning Sync Library off for all of your devices, then turning it back on again.  Learn how.