If you have a management group that covers all subscriptions then you can assign RBAC role (or even better create custom RBAC if MS roles would be not good enough) and user will get access to all resources/resource groups/subscriptions in that MG.
But is that user really required to have access in Azure Portal or maybe just access to DB with SSMS/other tools is needed?
Anyway for that you have Azure AD Admin - if you have it enabled for group - and by any change that is same group along all your servers - then this will be easy to add him to that group.
If not - you can create new group with this user in it - and add group as user to all DB with required permissions - via registered servers in ssms - that would allow you to execute query at all Databases with just one click.