Dawn is right. I know your focus is on your dog just now, but please, talk to the police right away. Someone in authority needs to locate your FIL because if he could do this to a dog, he could do this or worse to a human being--or to himself. Even if FIL is a total jerk of all jerks, he's also legitimately sick (you said he was deeply depressed); can you imagine the lifelong issues for your husband if his dad is found dead in a ditch or arrested for hurting another person, especially if no one was told that FIL might be a risk to others? Better to tell the police all about this now, and say that it is likely FIL is not in control of his actions. The cops well know that people who hurt animals often go on to hurt people. And for a man who never even got out of bed except for meals to now be not only out of bed but out of the house and gone -- very scary. I think he may end up missing or dead. Right now I know you might not mind either very much, and of course he should never come near your child ever again, but I'd at least let the authorities know he's out there and potentially dangerous to others or himself.
I'm not saying take FIL in and help him yourselves, I'm saying tell the authorities he's out there and possibly dangerous.