Dubia roaches and hornworms are good options. Black soldier fly larvae are also good to include on rotation. Silk worms too, when you can get them (they tend to be seasonal).
Did the vet mention what the lump could be? Since you have paper towel down, it's not likely the substrate unless you've seen him bite off chunks of it (that would typically pass through without much issue though). Has he eaten any other objects that could be difficult to pass? I would keep a close eye on the lump, focusing on the size and position to see if that changes over time. It's possible that it will move through, just slowly. If it's not moving, but is changing in size, I'd report that to the vet.
Did the vet mention what the lump could be? Since you have paper towel down, it's not likely the substrate unless you've seen him bite off chunks of it (that would typically pass through without much issue though). Has he eaten any other objects that could be difficult to pass? I would keep a close eye on the lump, focusing on the size and position to see if that changes over time. It's possible that it will move through, just slowly. If it's not moving, but is changing in size, I'd report that to the vet.