Make him a human raised/trained by halflings.
(Use the Human bonus feat at first level to take Racial Heritage: Halfling) so he doesn't get hosed later when things get too big for him to trip faster than he gets his phantom size increase.
I had a build somewhere that at level 12 needed a 3+ on the dice to trip and pin an adult red dragon (CMD 45 or so).
Underfoot adept... Maneuver Master, Sensei, and Hungry Ghost monk (whatever gets the knockdown kick) 3 of them in some combination, I think stack... or they did at the time before the rules got changed.
You don't need agile maneuvers for a trip, since a trip can be done with a weapon, and weapon finesse applies to unarmed.
Combat Reflexes, Imp Trip, Gr Trip, then the grappling feats since the prone bonus to attack applies to maneuvers.
It was a very specific trick... but it works well. It just doesn't do damage, so most people will tell you it becomes worthless.