'Mamma who'? You've forgotten your mother already?!
He then went around improvising on that with everyone he could find, sometimes using his own name (with friends) and sometimes using the names of other people who knew each other. Here are another few of my inventions made in a sleep-deprived moment. (Due apologies if you've heard this before or if it's the oldest knock-knock joke ever.)
'Boo hoo' who?
Now what are you crying for?
'Hoot, hoot' who?
Stop doing that. Are you a boy or an owl?
PS: If you're reading this and have your own favorite knock-knock joke, do add it to the comments. It'll save me some frantic searching on the Internet (coz he still demands a new knock-knock joke out of the blue and leaves me stumped) and you will have my heartfelt thanks. :-)