I’m not aware of a two-way calendar synchronization app, but I’m not a Calendar guru 
Maybe other will provide you with a app solution, meanwhile we can figure how to get your husband’s appointments 
For web site only (will come back later for phone), read-only one-way :
On Google web site, click three-dots button next to the calendar name, then “sharing” :
Scroll down to “Integrate”, and copy the secret URL (be sure to select the whole URL, with horizontal scroll) :
On /e/ web site, click the + next to “New calendar”, then paste URL :
You may have to wait a couple of minutes, then events appears :
