I think with the Hezrou's Endurance invocations you should do something like this:
"This invocation grants you a +4 enhancement bonus to Constitution as well as Damage Reduction 2/magic. At 7th level the Damage Reduction granted increases to 5/magic and cold iron. At 13th level the enhancement bonus to Constitution increases to +6. At 19th level the Damage Reduction granted increases to 10/--."
Basically, grant a small unique benefit in addition to the +4 enhancement bonus. Then scale the unique benefit up as well as the enhancement, but cap the enhancement at +6.
At first, I was thinking, "omfg, +4 to an ability score permanently at 1st level is so stupid and broken," because I was thinking of the Warlock just grabbing +4 to Cha. But then I realized that even if he does that he's getting almost no benefit from the invocation until 2nd level, and he could have gotten the same benefit from the Ability Focus feat. Now, the +2 to save DCs that can stack with the Ability Focus feat IS nice, but it also takes up the Warlock's enhancement bonus slot meaning he's not going to be benefiting from any standard items.
All that said, you should probably make the ability enhancers that benefit the Warlock the most (Charisma obviously leading the pack, followed by Strength, Dexterity, and Constitution in no particular order) have weaker secondary benefits than the ones that benefit the Warlock the least (Intelligence and Wisdom).