Since it looks like the CR maxes out at CR7 the DM could allow the CR to increase at every level that the Paladin adds a spell slot as follows:
Paladin Level 5 = CR 1 or less
Paladin Level 7 = CR 2 or less
Paladin Level 9 = CR 3 or less
Paladin Level 11 = CR 4 or less
Paladin Level 13 = CR 5 or...