Hello, I'm Dr. Kara. I have over 20 years of experience as a veterinarian and I'd like to help. Please give me a moment to review your concerns.
I am sorry to hear that Albie has been losing weight in spite of eating normally.
I am not surprised he is lethargic and quieter than usual if he is losing weight, as if he's not getting enough calories to maintain his weight then he will not have his usual energy and personality.
When we see weight loss with a guinea pig that wants to eat we need to worry about disease processes that either don't allow him to eat normally, or digest and/or absorb the food he is taking (such as diabetes, liver disease, primary intestinal or pancreatic disease including cancers) or those that lead to a higher metabolic rate such as cancer or a systemic infection.
Sometimes we can see weight loss with tooth problems (overgrown molars or root abscesses) because they are unable to chew and grind their food into appropriately digestible pieces. This would be gradual weight loss, and I would expect that you would hear him grind his teeth or drop food, especially hay, as he is eating. But given that his stools are smaller too it could be that he is mouthing his food more than really chewing and swallowing. That means his bowl may be empty but much of his food may be spilled and not really eaten, so this may be hard for you to determine.
Since he seemed to slow down on weight loss and/or actually gain a bit of weight when he was on holiday perhaps he was able to eat more slowly, over a period of time, and not compete for food when he was there.
Albie needs to see his veterinarian for another examination and some blood tests. Diabetes is a metabolic disease that we see with some frequency in these little ones. Normally we see an increase in water consumption and urination too, but it may be early enough in the process that he is just having trouble maintaining his weight. Diabetic guinea pigs do best on high fiber diets, so push hay for now and stay away from fruits.
I would also want to sedate him and fully examine his mouth and teeth for signs of an abscessed tooth root, spurs on his back teeth or uneven chewing surfaces.
Please let me know if you have any further questions.