I guess by now you took him to the vet.
White or very pale mucus membranes often mean there is an internal bleed (in the absence of obvious external bleeding)
It is very possible there could be a tumor on his spleen. Even "benign" (harmless) spleen tumors can cause internal bleeding. It is possible he may have to have his spleen removed.
This is major surgery, but most often completely successful, (especially for removal of a benign tumor) and if this is what's the matter with him, he should be very much better after 6 weeks recovery.
Sometimes if the bleed is a slight one, or a "slow bleed" the dog recovers ok once clotting starts, and the body will re-absorb the blood in the body cavities, but there can be some anemia and jaundice afterwards for a few days which may cause loss of appetite, tiredness etc. He will need a little more rest, and as much good food as he can eat.
Hi...The best advice I can give you, is to get your dog to the Vet now...Please do not waste any time waiting for help from this forum, as it sounds to me like you have an EMERGENCY on your hands. Your dog needs immediate care.
Please get him in to see the vet...let us know how things go.
I wish you the best...
Connie