i used to own a guinea pig stratus and my 3 biggest recommendations is
1st - once the little critter recovers/puts on weight & starts to get active, give the piggie as BIG of a living area as possible cuz they LOVE to do "laps" in their home...(a plastic kiddie pool with no slide is perfect). just put some bedding in there and a "house" for him to sleep in, amungst other safe chewable items for him to play with & a water source of course.
the 2nd thing is, guinea pigs bodies cannot naturally produce vitamin C so that is a supplementation need they have but it is easy to do, just make a salad of sorts with all kinds of goodies like veggies & fruits...
here is something that gives you a good food list to use...
guinealynx.info/fave.html
3rd - i was gonna get into the grass the little guy is gonna need but this will do just that for me...
guinealynx.info/forages.html
also here is something on behavior and the noises your likely to hear and what they mean...
guinealynx.info/behavior.html
there is one thing that is not mentioned there that i find necessary to share with you.
guinea pigs will make a noise that sounds like a high pitched "weeek". it is called weeking, sometimes whistling, although to my ears it is not a whistle. from my experience they do this when they're hungry and know food is coming, my piggie, as most do, figured out that his food was in a plastic bag and every time he heard plastic he started weeking, thinking he was getting food.
also i've heard my piggie do something that resembled grinding of teeth, kind of a clicking growl, if you will & since it sounded to me like a growl i took it to mean he was not happy. i may be wrong about that but to me a growl is a growl and it's distinctly different then the soft hum of a purr like one made by a happy cat for example.
well, i hope you find these bits of info helpful and i'm sure you'll find more by browsing the entire guinealynx.info site.
good luck with the little guy.