It sounds to me like he may have multiple problems. A size 6 popper is not big relatively speaking, not even for a 6 wt. I've used a 6 wt quite a bit for bass fishing, and even some Striper fishing. I have no problem casting flies, and even poppers on 1/0 or 2/0 size hooks. Now, I cannot cast them the same distance as I might with a heavier weight rod, so perhaps that may be the first issue, trying to cast too far.
If you can cast the poppers 30 to 40 feet, you're doing fine. If your attempting to cast beyond that distance, your likely creating part of your problem. Plenty of bass can be caught within that distance, so you really don't need to make long casts.
Next, the Cortland 2X leader you're using, you didn't mention it's length. A 7 1/2 leader should be OK, but if it's 10 or 12 feet, then that may be the next issue. You don't need long leaders or long tippets with poppers & especially not with Clousers. I've used 6 to 8 ft leaders for years with my 6 wt, but my leaders for big flies are as heavy as I use with my 9 or even my 10 wt. A 2X is a trout leader, not a bass leader. It's nearly impossible to efficiently transfer line energy with big wind resistant poppers, or heavy Clousers with a trout leader intended for tiny flies, especially if it's way too long.
The rod wt really is not the issue. A 6 wt line is sufficiently heavy, but will limit distance. Look for a leader that has a butt diameter approximately the same as the connection point of your fly line, and only about 6 to 8 ft long maximum with tippet. 0X or at least 15 lb test tippet is a good start. I've even used 20 lb test at times, but my flies are a lot bigger than size 6. Again, this is not dry fly fishing so you don't need finesse! It's about transferring energy to make the cast with bigger flies! Plus bass are not usually leader shy.
Even a furled leader might help, but it should be for a heavier weight rod, like for an 8 wt.
Last issue, casting! If your casting is fair to good with trout flies, then you should still be able to cast bigger flies. But, you do need to change your casting stroke & timing. You won't get nice tight, picture perfect loops like you might with trout flies. You will need to slow down some & open up the loop with the poppers & it will be a slower stroke due to all that wind resistance. With Clousers, it will be more of a lob than a true cast, and you'll likely get even less distance than with the poppers. Folks call this chuck & duck casting for a reason!
Tight loops & Clouser Minnows, or any large, heavily weighted fly with barbell, cones or even beads is a great way to break your rod, or impale a body part. Actually, the impaling is preferred as long as it's not in your eye, as that will heal! The rod won't. Barbless or de-barbed hooks might be a good idea for you!
If your casting sucks with trout flies, then you need to work on it anyway, and should get some help from someone who regularly uses the big poppers & Clousers! Big difference between casting bass size flies & casting trout flies, especially with a 6 wt.
One more thing, learning to double haul is not a bad idea, but you don't need it to catch bass!