No credit card required
Browse credit cards from a variety of issuers to see if there's a better card for you.
If the account shows up on his credit report, yes his a FICO score will be generated and yes, his AAoA will go up. I don't have much experience with AU accounts, as I've never been a part of one either by adding someone or being added. My understanding though is that there are cases where AU accounts don't show up on the AU's report for whatever reason. I think one of the reasons could be the relationship between the primary and the AU. Some creditors like to see the same address, last name etc. to sort of prove a relationship, as in the past AU's were abused with people on one side of the country basically adding someone (and being compensated for it) that they had never even met. I know some of the veterans on here can weigh in far better than I can on this topic though.
Good responses by the other folks. I notice that you describe the mother's card as "well managed." Two quick points:
(1) If by that you mean that there has never been a single late payment, that's encouraging. But if there has been even one, then your partner will be going from a clean profile to a dirty one by being added as an AU.
(2) You don't tell us whether what the balance is compared to the credit limit. Some people would consider a $500 limit card that they run $400 each month on as well managed, because they are paying it off each month. From a FICO utilization perspective, however, that would be considered bad, since the card looks like it is always close to being maxxed out. So an important question is whether the card is usually reporting a very small balance compared to the credit limit (less than 9%).