Hi ... I have the same problem, and it drives you up the wall. I wear a Scopoline (sp?) patch behind my ear, which cuts down the amount of saliva, but doesn't thin it out. In fact, some medical type told me that it may contribute to the problem, because sometimes thick saliva may be caused by dehydration.
One solution I have heard many times is the use of meat tenderizer on a swab or a Q-tip. Seriously ... meat tenderizer is an enzyme (natural stuff, I think it's from pineapples). My problem is that I can't find it in the stores anywhere. I've been told this by doctors and therapists.
I used to use Musinex to thin the secretions, but I can't swallow the pills any more. This seemed to help thin things out. There is a liquid form for children that I am going to try in my tube. It's vile tasting, but my tube doesn't care.
Part of my problem (don't know if it applies to your dad) is that when I eat thick things by mouth (yogurt to take pills), they do not stay swallowed. They keep sliding back into my mouth and mixing with the saliva making a VERY unpleasant glue-like gunk as you describe.
I also swab my mouth out with paper towels. With me, if I can get it stopped, it will stay stopped for a while. My thick saliva comes from only one part of my mouth ... the left side toward the throat. I have heard that some people get Botox injections to "turn off" particular saliva glands, but I've also read some PALS accounts that it did not work too well.
Good luck. I hope you find a solution for your dad. It seems that with all the powerful things going on in our bodies with ALS, something like saliva should be the least of our worries, but in fact it really affects quality of life.