Guinea pigs are prey animals with survival instincts to hide illness or pain so guinea pig health problems and diseases are usually not noticeable until their advanced stages.
Cages / Feet
Cavies (guinea pigs) do better in cages with solid floors. Wire mesh floors can injure their feet and even cause foot infections such as bumblefoot. This is one of those guinea pig diseases which is easy to prevent. Just get cages for your pet with solid floors.
Toxemia of pregnancy is common and kills many females. Signs include sweet smelling or fruity breath due to ketones, loss of appetite, lack of energy, and excessive salivation.
Their teeth like many rodents never stop growing so in attempt to prevent their teeth from getting too large for their mouth they are constantly gnawing which lessens the length. And also for this reason you need to supply his cage with things he can chew on.
Guinea pigs actually feed fairly consistently (how nice is that? The rest of us would swell up) so a constant supply of hay or other food is recommended.
They like alfalfa and will eat large amounts if it is given to them and it is a source of amino acids, protein and fiber. Some think excessive alfalfa can lead to obesity or even bladder stones because of the calcium so that may be something to watch out for.
Neither cavies or humans are able to make their own vitamin C when most mammals do. And like humans stripped of vitamin C they will get scurvy which can be fatal to cavies. Signs for this could be joint stiffness or they can just get sluggish. They need to get their C from fruit like apples or vegetables like cage, spinach, celery and carrots or a daily supplement.
They also need vitamins D, A and E as well as certain minerals or can get vitamin deficiencies or muscular dystrophy.
But do not make any drastic changes in your pet's diet. Cavies do not do well with sudden changes in diet and in fact may even stop eating and starve.
If your pet is scratching excessively he may have a parasite. You can brush through his hair searching for them they are small and brown in color. You can get a special shampoo from the vet which will take care of it. You will also need to clean and disinfect the cage.
It could have been torticollis or wry neck which is usually gone in a day. If he is having problems walking with it as well it could be a middle ear infection which could potentially damage his hearing so you would need to take him to the vet.
With most guinea pig diseases it's best to take your pet to the vet preferably one who specializes in exotic animals.