So much about the Demonologist class appears to be designed around its ability to summon and command demons in battle; it feels like this should be the class' raison d'être. Based on this, one should get the sense that the Demonologist is objectively the best summoning class out of all of the classes with the ability to summon... but this hardly seems true. In fact, with summoned demons being able to break free of control after taking sufficient hit point damage, and with there being no way to heal summoned demons, one wouldn't fault a Demonologist for withholding their demon summoning spells more often than not. Which just feels wrong; you should be encouraging your Demonologists to show off their stuff by summoning as often as possible!
With this in mind, I offer the following modifications to the rules for a Demonologist's summon spells:
Degradation Mechanic: Degradation works mostly as stated in the Book of Demons, with the following exception: degradation does not affect a summoned demon's hit points.
Instead, a summoned demon has a secondary resource pool, comprised of control points. When first summoned, a demon will have a control point pool equal to its hit point pool, and when the demon suffers degradation, the degradation will consume its control points and not its hit points.
Loss of Control: When your summoned demon's control points drop to 0, the Demonologist rolls an easy save (6+). This save is +5 points harder if the Demonologist is unconscious, and is also +5 points harder for each extra demon the character has summoned beyond the first.
If the save succeeds, the demon is simply unsummoned and returns to the Abyss. If the save fails, however, the demon breaks free of its summoner's control and becomes an enemy, attacking by preference its former summoner, though it may fight somewhat intelligently to do as much sadistic harm as possible to the summoner and their allies.
Demon Healing: You can heal your summoned demons just like any other allies in battle. Usually, when your summoned demons heal, they heal using your recoveries.
Maintaining Control: More than healing hit points, your Demonologist will likely want to try staving off degradation and staying in control of his/her summoned minions. That's understandable. But it requires effort on the Demonologist's part. Once per battle, the Demonologist can spend a quick action to Maintain Control; otherwise, the Demonologist must use a standard action to Maintain Control. Mechanically, this is similar to rallying, except instead of spending a recovery to heal yourself, you are spending a recovery to replenish your summoned demon's control points.