EDIT: Question about my sick dog.

I have 3 dogs and my oldest fur baby is 9. He has lost a lot of weight very quickly. He eats like a pig but still no weight gain. Could it be worms? And if he has worms will my other dog get worms from him? We plain on taking him to the vet next month when my husband's bonus comes in so don't worry he is going. I just want to try and fix the problem before then if I can so there is no outrageous vet bill. If it's something as small as going as picking up a bottle of worm medicine. But my dad told me if he has worse they all have worms. Is this true? If one has it will they all have it? I feel so stupid even asking. I just want to fix the poor guy now.

EDIT: We wormed all our dogs just to be safe but still gonna take our old boy to the vet next month. I called the vet and I know him personally. He told me it could be old age and it's his way of telling us he might not be here much longer. Not to panic the way it's goes. I didn't tell my husband this because that's his baby. The vet wanted me prepared because he knows the bond my husband has with Bandit. Keep us in your thoughts please.