Two thirds of all cuckold fantasies, which are extremely common particularly among men with above average education levels and successful careers, feature embarrassment or humiliation through penis-size comparison (other man larger and more satisfying, seen or said to be so). An Australian researcher named Angela Lewis did some interesting research on this of men in the US and UK.
As to the exact words, they are highly individual as to what really flips his switches. Use a teasing tone and mix it up. Alternate small, tiny, little, and similar adjectives and their modifiers such as extremely, exceptionally, so very, ridiculously, and laughably. Use the words unsatisfying and inadequate. If he's up to it, tell him that you told a girlfriend of yours that he's got a small one. Experiment, have fun with it, and see what gets him going the most. Because he's asked for it, you can be reasonably sure that it isn't going to hurt his feelings or get him angry.