Cats usually only drink water, unless they are still kittens. You can get cat milk for them as a treat, but it isn't good for them to have it every day.
Have you had your cat checked over by a vet? If he has plenty of food but isn't putting on weight he may need to be treated for worms. You should also have him checked for diabetes and hyperthyroidism. Both of these conditions can cause weight loss.
Once he has a clean bill of health you can try getting him to put on weight through diet changes. What is he being fed now? Could you tell us a bit more about your cat, age, weight, whether he is an indoor only cat or whether he goes outside etc?
Has your cat always been skinny or is this something new? Some medical conditions can cause weight loss such as throid disease and diabetes. Only a vet can run bloodwork to determine if your cat has a medical condition and start the proper treatment. Dental issues can be painful and result in a cat not eating much. A vet would have to determine the extent of the dental issues and remove the bad teeth.
How much of the dry and canned food are you feeding every day? Friskies pate canned is fine but the Kit and Kaboodle dry food is horrible junk qualiy food, like a greasy high calorie and preservative laden fatty burger from a fast food place. Does your cat actually eat or just picks at food every day?
There is no liquid drink you can give to your cat to put weight on. It doesn't exist. Kitten milk replacer may provide some extra calories but it won't solve any underlying health issue your cat may have.
Your cat really, really needs a check up as he is a senior. He could have a thyroid condition, etc. Please use some of the links above to get him some help. Diet will not fix this alone.