The lettuce will eventually soften up and decay within a day and he'll probably work it out of his mouth.   If you want to try and remove it , you can net him and hold the net close to the surface of the water and pull the lettuce out  when you see an opportunity. Just make sure he can't jump out of the net and injure himself.

It's okay Lucky's done similar things XD and I have freaked out.

Once she had a piece of pea sat in her mouth, I could actually see it there and was worried she would choke, it was gone after a few minutes. Then the other day she bit off more than she could chew, she struggled a little, chomping like crazy and I was like 'this is it, I've killed my fish with a pea!' then she spat it out and ate it back in again, she did this a few times, till I managed to get in there and bat her away from the pea. But noticeably the piece of pea was a lot smaller, so I guess eating it in that fashion was easiest for her  
I just panicked because it looked like she was struggling, but she knew what she was doing

If anything ever does seem stuck, I've read about people using tweezers to get things out. Plus with the fish so used to you, you could always gently hold them or lift them out of the water and try to remove what ever it is.

Mine tend to grab the hard pleco pellets. They're just about the size of my biggest fishes' mouths, I usually just let them chew They're just fine since it's food and it softens up eventually...
Once they managed to catch 5 of them though, and I just thought that looked too horrible so I made them spit it out Simply because it looked like it was keeping them from breathing normally (They kept 3 of them though...).

In case you'd one day notice something really being stuck in the fishes' mouth/throat, like a rock or something, you should gently take the fish and push the obstacle out through their gills with something thin, but not sharp. In case your fish start floating around suddenly, that could be a sign that there's something stuck and they can't control their level in the water.