First, I have to say that I am terribly concerned about your lad.
While he is refusing food, I am worried this i snot due to a GI upset or similar issue. Instead, it is those white gums that are ringing alarm bells. This is a sign of low blood volume or poor oxygenation. It could be related to issues with getting enough oxygen into his body (due to heart or lung issues) but also could be a sign of internal bleeding or anemia. In all cases, this will make a dog feel weak, lethargic, collapsed and many will refuse to eat and drink (especially if they are struggling to breathe fast enough to get oxygen to just survive).
In this case, this is an emergency. I appreciate he is a large dog, but you will either need to get a sheet/towel/rug under him to lift him to the car or consider ringing the local vet for a home visit. In this situation, he needs to be assessed and treated urgently before this can progress any further. Depending on his vet's findings, this is hopefully something that can be treated (ie using blood transfusions, removing a bleeding spleen if present, getting him on oxygen) but I do have to warn you that his prognosis is guarded as he is just now as he is. Therefore, to give him any chance, we need to act quick and not let deteriorate further.
In this situation, it would be prudent to get your wee one to the emergency vet. To find your local ER veterinary clinic, you can check @ http://www.vetlocator.com/or http://www.veccs.org/index.php?option=com_hospitals&nationid=1&Itemid=193
Please take care,