Using fleece:
https://www.guineapigcages.com/forum/threads/21568-The-Fleece-Project-The-Study?highlight=fleece
Guinea pigs need at least 10% of their body weight in veggies every day. You need a kitchen scale to weigh Picasso every week on the same day as well as his veggies every day. Usually, the first sign of an illness in a guinea pig is weight loss so if you notice that Picasso has lost 50+ grams or a few ounces in just a few days or 1 week, you will know something is wrong. Keep a journal to record his weights every week for reference. Kitchen scales can be guinea pig life savers; it's an essential thing to have when caring for guinea pigs. You'll want a scale that measures in grams or ounces.
Lettuce and bell pepper should be the staples. He needs 1/8 of a bell pepper every day for his Vitamin C; guinea pigs can't make their own Vitamin C, just like humans, and they can get scurvy if they don't get enough Vitamin C. Bell peppers are rich in Vitamin C and they are the easiest and most common way to get the Vitamin C in a guinea pig. Make sure to include a large variety of veggies so he gets all of the essential nutrients - you can google what he can eat and what he can't. Here's a small list of what they CAN eat DAILY:
-Lettuce
-Bell Pepper
-Cucumber
-Carrot
-Tomato
-Endive
-Raddichio
There's much more. I'd encourage you to research. If you go to the search bar on this site and type in "veggies", many threads will come up on what kinds of veggies to feed, how often, how to get a pig to eat them, etc.
I would try leaving some veggies in his cage and see if he eats it. Leave it in his cage over night if you have to. Put them buy his house so he doesn't have to come out. If he doesn't eat today or tomorrow, just keep trying. He'll learn to eat veggies eventually.
I'd really encourage you to get Picasso a buddy. Guinea pigs depend on one another for security and most of the time they are tamer and more confident around their owners when they are living with their own kind. It's actually ILLEGAL to own only one guinea pig in Switzerland, did you know that? Most pigs benefit a lot from a having a buddy or 2. Guinea pigs can even become lonely and depressed if they're alone and they can stop eating. Guinea pigs are healthier, happier, more active, more confident, and tamer when they have a friend(s). They are natural herd animals. Some pigs just can't get along with any pig, but that's extremely rare. Unless you've tried Picasso with multiple guinea pigs, you can't say he's better off by himself.
Here's some info:
https://guinea-pigs.livejournal.com/2982433.html
https://www.cavyspirit.com/sociallife.htm
(broken link removed)
How to bond guinea pigs successfully, the full guide:
https://guinea-pigs.livejournal.com/3002707.html
Please adopt Picasso's friend, don't buy another from a pet store or breeder. They are usually very sick and most have bad genes because of the way they are bred. Pet stores and most breeders know nothing about the proper care of guinea pigs and they are very cruel to the animals. By buying any live animal from a pet store or breeder, you are supporting the cruelty. There are thousands of guinea pigs of all ages, colors, and breeds in shelters and rescues; we can help you find one around you if you are struggling. You'd have a much better chance at getting a happier, healthier, longer-living animal from a shelter/rescue rather than a pet store/breeder. Shelters and rescues pay for vet bills, unlike pet stores who don't care and just keep the sick ones "in the back", and breeders who just want money and don't care either. Shelters and rescues won't let ANYONE go home with a sick animal... pet stores and breeders don't give a crap. To them, you bought the sick animal (without knowing most likely), now it's YOUR problem.
There's also Craigslist, petfinder.com, guineapigfinder.com, and more. If you're lucky, you may even be able to get a pig for free on places like Craigslist. As long as you prove that you have a guinea pig who needs a friend and you have a cage and everything, they may be able to give their guinea pig to you for free or at least lower their asking price.
Usually, the reason that people put a price on their ads for their guinea pig(s) is that they don't want to attract people who are looking for free snake food. I got my boy Bramley for free along with some of his stuff on Craigslist after asking a lot of questions and proving that I had another piggie at home along with a cage.
Speaking of cages, please be sure to check out the main page of this site for cage size standards. One of the most common reasons guinea pigs don't get along is because of the lack of cage space. If you have a pet store cage, you'll have to get rid of that. No pet store cage is big enough for 2+ guinea pigs, let alone one. They are ridiculously small and they are ridiculously expensive. There are many alternatives; take a look at the Photo Gallery, you're likely to get inspired! And remember that bigger is always better, so go as big as you can. The bigger the cage, the more likely Picasso and the newbie will get along (However, it's their personalities that determine it all). Also keep in mind that boars need more space than females so if you're getting Picasso a male friend I'd suggest at least having a 2x5 cage (28in.x70in.), if not bigger.