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As a temporary workaround can you set it up with your device, then switch the account it is signed into for music? See the Manage Music & Podcasts section of Change settings for HomePod - Apple Support. Try signing out of your Apple ID there, then have him sign in with his. If the setting in the Home app is to allow speaker access by anyone on the same network then either of you should be able to AirPlay to it, and he would have access to his purchase history, and iCloud Music library if he has Apple Music or iTunes Match.
Question: HomePod only with iPhone 6s and not iPhone SE