How to deal with a player who knows way too much more than the DM?

No.49207700 ViewReplyOriginalReport
This is in D&D 5e

TL;DR at the end.

Ok let's get some backstory, there are 4 players and then me, the DM, we'll call the players Y, W, B and S.
Y & W are playing D&D for the first time.
B has played a lot of D&D over several years.
S was playing D&D for the first time, then went off on a trip with friends after our 2nd session and did 2 weeks of playing almost nothing but D&D for 8 hours a day. And they played D&D pretty much -every- day.

I've played D&D Pathfinder a little bit and I've never DM'd before.
Ok I'm a new DM, I've only DM'd 4 sessions ever with this group.

Our session is a premade campaign (Lost Mine of Phandelver) and it has sort of evolved into teaching Y & W how to play D&D and teaching me how to DM entertainingly. B was there mainly to help set up character sheets and advise on how I should operate as a DM at the start, but ultimately left it up to me.

However, since S has come back he rerolled his character from a Bard into a Sorcerer and it's sort of thrown a wrench in the campaign because he's suddenly able to overcome any of the challenges with a slew of new abilities which just make his character sound like some silly comic book hero power fantasy, such as having a familiar that can shapeshift into anything, the familiar being able to turn invisible and immaterial to pass through walls, and said familiar able to cast any of his spells as well as telepathically communicate with him - at infinite range.
He said that the trade-off to this would be being less strong in combat but he still dishes out a very good amount of damage.

(cont. - pretyped)