"How does one go about getting a toddler to sit still for a haircut?"
Tequila. For both of you.
Honestly, though, this is one of those things where getting him very tired and doing it while he's asleep isn't a bad idea, assuming the trimmer doesn't wake him. If it does, then get yourself a nice sharp pair of scissors and keep him up late, and as odd as it sounds it works pretty well. At least it did for my oldest.