Buy a doggie sweater or clip the coat a little longer than usual. Give your senior dog a gentle massage as often as possible. Not only will it soothe and relax both of you, it will help you find any lumps or injuries that need veterinary attention. It's especially helpful for aching joints and muscles of the senior dog. Step up vet care - teeth, eyes, nutrition.
Dental care is vital.
Dental infections can stress the system and even contribute to major organ disease. Police the food bowl. Younger dogs may take advantage and hog the food. You can feed your older dog in a separate room or in his crate to ensure he is getting his full share. Watch the weight.