Hello. I believe I can be of help to you with this issue.
The symptoms you describe could be many things---a unprescribed medication, or a poison of some type. Routine blood tests won't reveal what might be in his blood stream. At this point, I would contact his father's physicians and ask for a brief appointment. They will tell you whether they are also suspicious that some foul play is occurring. You can share your concerns and suspicions and ask them to at least contact the police to let them know what is happening. Explain that if your husband dies and it is later found that the medical staff treating him had been alerted to the possibility of poisoning, and never did
anything about it, they could be held liable for damages and malpractice. So they actually do, legally and ethically, have a 'duty to warn' the police (and I'd make sure I use this phrase as it has legal meaning to all health care providers---"duty to warn"). Tell the doctor that you are willing to be interviewed first, if need be. Then, if the interview occurs, tell the investigator you feel it is very important that they tell your stepson that his dad's blood work and health decline appear to be very suspicious
and that if the man happens to die, there is almost no chance that any inheritance or money will be distributed from the man's estate. You'd want to ask the police to try to find out if your stepson has a life insurance policy taken out on his dad that you don't know about.
I'm suggesting the above steps because it is important for you to be somewhat protected from the stepson knowing that you initiated this investigation. It should be initiated by the doctors. If nothing else, you can explain to the police that even if nothing is found in the blood work (i.e., the police can probably have your husband's blood or hair tested for poisons---which routine medical blood tests won't pick up at, say a clinic or hospital); IN THE VERY LEAST the stepson will now know that he has to stop what he is doing with the poisoning, or else he stands a good chance of being discovered----that is, he will have to suspect that the police are watching this situation and will consider the man's death suspicious. What I'm saying here is that merely having an investigation right now can protect your husband's life in the future because the stepson will stop doing whatever he is doing out of fear of being caught. What do you think?