I wouldn't say that my boyfriend has been trained to last any longer (thank god!), but I know that having so much stimulation can train guys. Like JR recommended, cock rings should help him last a bit long as well as wearing a condom would as well.
As for stamina, Fleshlight does make a Stamina Training Unit (STU) that I've heard a lot of positive reviews about how it increases how long they can last.
It should fit smaller guys. If you find it's a bit loose, there's Fleshlights that are made to be tighter that should fit him fine.