A few minor things first:
He's not sure it is possible, in 1h+shield mode, to drop the weapon, cast the spell, and pick it back up. Is Jon correct?
As far as I know, this is incorrect RAI (but not RAW): Dropping an item does NOT count as an Item Interaction*, so Jon could still pick up their sword as a Free Object interaction during his move.
* Apparently the original tweet, and thus the related Sage Advice entry, has been deleted. At one point, there was a direct ruling that a player could drop + pick up their weapon in the same turn without using an action. I don't know if that ruling was contested, or if the tweet just isn't available anymore.
As DM, personally, I'd probably roll to find out if the sword rolls away or falls on his toes though, so that's not an ideal solution under those conditions (not RAW, though).
Jon thinks he should start by using a Component Pouch, to avoid having to waste "free-object-interactions" to get his Material Components (as seen here). He thinks it might be possible, when in 2h-mode, to release one hand, cast whatever, grab weapon again.
With a 2handed weapon, this isn't necessary. It's perfectly legal to hold a 2handed weapon using one hand; most of these weapons aren't that heavy. The requirement for these weapons only applies when actually using them. So releasing one hand, grabbing your focus, and casting, is perfectly legal, regardless of the circumstances:
A Spellcaster must have a hand free to access [material] components, but it can be the same hand that he or she uses to perform somatic components.—PHB, section on Material Components in Spellcasting Chapter
Note that there is no need to have a Component Pouch. Incidentally, Component Pouches don't meaningfully change the rules for spellcasting; they just function as flavor to make Spellcasting more interesting.
So, how can Jon alternate between casting and attacking without dropping his awesome weapon on the ground for others to steal?
As mentioned, for two-handed weapons, Jon is covered. And for Sword + Shield, Jon doesn't usually need to worry about anyone swiping his weapon (beware of readied actions, though!)
A better solution would be to not have to drop it in the first place:
You can perform the somatic components of spells even when you have weapons or a shield in one or both hands.—PHB, page 170, Warcaster Feat
You can use any weapon you summon with your Pact of the Blade feature as a spellcasting focus for your Warlock Spells.—Xanathar's Guide to Everything, page 57, Improved Pact Weapon Invocation
If they already plan to take the Warcaster feat, then they would only need to take the Improved Pact Weapon Invocation to be able to perform Somatic + Material components of their spells without needing to drop or even let go of their weapons. So Jon might consider just swapping one of their Invocations for IPW next time they level up.