Johnny can only do what the description says. He can communicate telepathically or depart for the afterlife.
In a few cases, communication is sufficient to do things. I mean, you could teach Jack about an ancient earbug song. Or you could mislead Jack into entering a Lich's trap in exchange for the Lich helping you get a body.
There is even the UA Diplomat feat that grants you the ability to charm someone by talking to them; telepathic communication would count as talking in any reasonable sense. Charming doesn't do much here, other than grant you advantage of further social skill checks and make Jack unable to harm you (charm in 5e isn't charm from 3e, it is highly limited). Similarly, you could perform a song using Performer and distract Jack with a disadvantage on Insight and Perception checks.
If worn by an animal, the Animal Handler feat (from the same UA) would let you command the beast. Your inability to see what is going on makes this less than useful, however.
Johnny has no ability to cast spells, no ability to maintain concentration or do actions (the Diplomat ability isn't an action, however). Anything that isn't an action and can be done by talking or communicating with someone can by done by Johnny, and only on Jack.