Hi Jayachbee. Thanks for posting.
I'm sorry to hear about your situation. If problems like these suddenly arise I think you might be spot on with the thyroid problem idea. However for that you'll need to go to a vet to see what exactly the problem is and what you can do to fix it. It might cost some money, but I'm sure you and a father to this family would rather keep them safe and use some money rather than having the unthinkable happen.
Bitting dogs is no small problem and they need to be dealt with quickly so the problem doesn't escalate. Also old dogs tend to loose their hearing and seeing and because of that they start becoming a bit snappy.
Correction is probably the last thing you want to do with an aggressive dog that bites. It only teaches them to fear you more and will usually result in serious bites. Its better to avoid anything that causes the aggression and find a way to fix what does cause it.
If nothing is physically wrong with the dog and this is just a behavioral problem you can try building a bond with this dog. I've heard of many cases where a dog is aggressive to family members and taking that dog for walks and hand feeding them seems to work wonders.
So instead of having people the dog likes, take him out for walks, try having the people he doesn't like take him. It'll help him associate them with pleasant things. Also instead of feeding him in his bowl (and risk food aggression) try hand feeding him. Just measure out what you would usually give him and feed it to him directly. This way he learns humans = food.
Well thats all for now. Here are some links that you might find helpful:
http://k9domain.org/alpha_theory.aspx - Will teach you about why corrections aren't the best idea.
http://k9domain.org/fearful.aspx - While your dog may not be fearful, many of the techniques outlined here should be helpful in dealing with aggressive dogs too.
I hope your situation gets better. Feel free to post updates or more questions you may have.