The major issue here is the fact that every resurrection spell in 5e requires a willing soul. Resurrection, Reincarnate, True Resurrection, and Raise Dead all require the spirit to be a willing participant and have a desire to come back to life. The only spell that can reverse death and doesn't specifically state that a willing soul is necessary is Revivify, but that requires the spell to be cast within 60 seconds of death.
The party isn't going to bother finding the resources to cast one of the resurrection spells on someone who, as far as they know, doesn't want to come back to life. It would be a major waste of money, time, and the spellcaster's abilities -- if they can even find someone able to do it, which can be difficult depending on their location.
As a result, to do something like this is the definition of metagaming. He learned something out of character (how much he needs his old character), that he couldn't possibly know in character, and now wants something to happen in character as a result (his resurrection).
It basically just comes down to how much the party is willing to tolerate stretching the unspoken "no metagaming" rule, but there are a couple of ways you could pull it off if you want to allow him to come back as that character.
Another letter gets sent.
Maybe, after all, living with Mom isn't such a great thing. He longs for the freedom he had when he was alive, and wishes he could return to adventuring; the afterlife just isn't all it's cracked up to be, and he regrets leaving his friends behind in a time they need his abilities. Mention that he would be willing to rejoin them, should they attempt to resurrect them, and then it's just a matter of letting the party do so.
The above, but other party members begin to see his ghost apparate and spook them. They get nervous, so they attempt a resurrection to bring his soul back into his body rather than leave it as an increasingly frustrated ghost.
They visit him in the spirit world.
You don't mention what the party's makeup is, or their levels. If they're sufficiently high enough level, you could make it a plot in which they get to visit him in the spirit realm. Perhaps he pesters the goddess of death into letting them visit, or they get her attention through a certain act and she wants to have a chat with them.
Just... do it.
Alternatively, maybe your players don't particularly care and just want to play, metagaming be damned. In which case, whatever. Just tell them to go find someone to resurrect him and be done with it. You should be very careful with this solution, however, and make sure you have a good read on your party, because something so egregiously metagaming could easily be a game ruiner for someone really invested in the story/world.