He may need a 2 level dip into fighter and/or PW for the feats if he intends to use Dire Flail Smash, as it is a bit feat intensive.
+1 Aptitude Keen Small Heavy Flails and the Feat Lightning Maces imo.
If your using Aptitude then you should just save your feats and go with Boomerang Daze as it can have a way higher DC. 10+ 1.5*Strength Mod+ Weapon Enhancement+Knowledge Devotion+Weapon Spec line+D-Damage reduction
I was going to suggest Staggering Strike, as the DC for it is also damage based, which is of course ricockulous.
Lightning maces gives extra attacks on crit threats, not dazing. I suggested small heavy flails because they are the only core 19-20 threat range bludgeoning weapon. Theres a Cestus floating around out there with a 19-20 as well. Only suggesting bludgeoning weapon because the OP mentioned a fighter, thought he might be going for Melee Weapon Mastery (bludgeoning) and various other feats that might chain off it.