Why didn't he just establish a council on top of the three dukes, put the dukes in the council but stuff it with loyal promoted officers from lower levels? The dukes get to keep their roles and titles but it would be de facto subsumed by the council as a whole, especially over time.
Case in point, see the central military commissions established by communist states (esp. PRC and NK) to keep party control over the army.
Also the former king not cooperating is very strange and definitely makes the situation a lot harder than it needs to be. He doesn't even need to do anything of actual substance, he simply needs to be present at meetings or give occasional speeches and visitations stating his confidence in the new king.