can i add another phone to my account

Not if he is using a different ID. You will need to know his Apple ID and password so you can use his ID for Find My Phone if his phone is lost or stolen.

Find My Phone - If your device is lost or stolen.      

Find My Phone - Lock and track your device using Lost Mode.        

Your husband should be using his own AppleID on his own devices. Sharing an AppleID will inevitably result in merged data that you don’t want on each others devices, or one of you will inadvertently delete something the other wanted to keep.

The two of you can always use family sharing and can share your locations with each other. To use Find My yo find a lost or stolen device , he would need to use his AppleID and his AppleID password to find his devices.