Welcome to the family!
Not necessarily. He might have been invited to help the old, ineffective remaining mages. Maybe because they are facing some issues that only a Gentle Gifted mage can overcome ?
Or because he ingratiated and was invited as a favor - one of the remaining mage suffers from a strange curse (possibly a bad Twilight episode) that can only be soothed my music (between Free Expression and Puissant Music, it will play right in the Jerbiton's hand).
Another alternative, one of the remaining mage is one of the last representative of a school of singing mage, a bard of sort and is looking for a heir, both to inherit the covenant, but also his legacy and duty as master bard.
Depending if you have some additional book like Tribunal (Normandy or Hibernia for example), House (Societas in your case) or The Mysteries, you can build on it and draw some inspiration as a goal to achieve within 15-25 years.
Don't fully stats the remaining mages, old mages are tedious to fully develop, unless you want some practice exercise. However having a few lines describing their house, specialties or pet topics can be useful, as well as how quirky they became with age (how close to final Twilight, Decrepitude or insanity are they?).
Although vis is useful, I would suggest you think more on what kind of adventures your mage wants to take part and what he does not want to do.
Take boon to avoid the task/adventure he does not want to do (if he does not want to hunt for vis, then yes, having a large collection of vis is nice, but are the remaining mages - who need powerful longevity potion - going to let him free access to the vis store ?) and take hooks for those he would like to have.
It will naturally prompt adventures towards what you are looking forwards and remove or alleviate the part you will find annoying for your mage.
And a useful link where you can find a ruleset for soloplay. This ruleset has been fully tested and even if I am still tweaking it, it works well for the first 20-30 years if you start straight out of apprenticeship.
If you use this ruleset, I would suggest to build your covenant as a Spring covenant within the frame of a Winter covenant. Let me explain: as the youngest magus, you do not have access to the full resources of the covenant, thus, the mage has access to resource more akin to a Spring covenant, but as he earns favors and make himself useful or even indispensable to the older magus, he slowly gets more access to more resources, or discover that they have not been well managed and have been squandered by decades of neglect and carelessness: rotten tomes of once-excellent Summae, slow plundering of silver coffers, etc...
So when your mage earn rewards, instead of coming from outside spoils of victory, he is getting clue to where was stored a pile of labtext and tractatus, or a mage remembered that he had been using a tome to level his uneven lectern, etc.
Your mage can spend half his adventures trying to run a dilapidated covenant to make it functional enough to attract other mages and the other half scavenging and retrieving lost knowledge from sanctum of long dead mages, but with fully functional defenses, live traps and even a ghost or two. A winter covenant can become a target from competing covenant - so politics to secure alliance and protection would be needed - but also from marauders led by edge wizard keen to plunder resources or even claim the site for themself if they believe the defenses have crumbled.