I love pranks. I can think of a few good ones.
%1. Get lots of googly eyes and stick them on his toys so that all his toys have two pairs of eyes. It'll really confuse him.
%2. Swap the sugar for salt so that his tea will have salt in it. It'll be disgusting. Or, if you're not feeling that mean, you can swap the salt for sugar and his chips will be really sweet lol.
%3. Put something sticky on their phone and then ring them. They'll get the sticky stuff all over their hand when they try to answer it.
%4. Put socks in his shoes, then when he tries to put them on he'll think they're too small.
%6. Put cling film on the toilet seat so that when he goes to the toilet it goes everywhere.
%7. Swap hand sanitizer for glue so that he gets glue all over his hands.