Cut the cake into parts equal to the number of people who will be eating it, individually wrap it and put their names on it. For him you can bring him a little toy to play with, or something he wouldn't mind eating without guilt.
Edited with the following on Feb 26,'19
Suggested toys: Tomagatchi, Fidget Spinner, Slinky (multicolored), Foam Dart Gun.
Possible food alternatives: Jerky or Jerqee (the latter is vegan), a veggie tray with dips.
I'm kind of getting the vibe this guy is diabetic.