How old are your dh's parents? 50's, 60's, 70's or older? I appreciate that you nc with your ils but would you be able to speak to your fil without your mil being present, and do you live near enough to be at their home within a short time if he texts/calls you?
Has your mil received treatment for her mental health issues, or have these not been recorded on her medical history as far you know?
Your fil's situation is not unusual and many men are reluctant to report the dv they suffer out of fear they won't be believed and that doing so may provoke the spouse to further acts of violence against them. The sense of shame felt by dv victims is common to both genders.
The problem is that social services, the police, mental health professionals etc are by and large powerless to act unless the victim, or the abuser, reports to them or they are called out by, say, a concerned neighbour or passer by when an incident is taking place.
I suggest that in the first instance your dh (as next of kin), or you acting for him with his permission, make contact with his dps' GP and report your concerns. This may be better done in writing and sent recorded delivery enclosing copies of whatever evidence you have of the abuse your fil is enduring.
I also suggest that you make contact with your ils' regional authority Adult Services and ask to speak to a social worker who is experienced in elder abuse, outline your concerns, and send them a similar letter to the one you've sent to the GP by way of a follow up.
While this may not in itself lead to any action being taken it will serve as a reference point should, for example, your mil accuse your fil of abusing her or if he should claim that any non-accidental injury he sustains was caused accidentally.
The Action on Elder Abuse freephone helpline can be found here www.elderabuse.org.uk/Mainpages/Services/services.html and they may be able to offer alternative suggestions that will enable you to worry a little less about your fil's plight.
I'm pleased to learn that your dh has benefited from therapy and wish you both well for the future.