okey first of all I think your new guinea pig sounds like he has a cold(a respiratory infection) which can end up making the guinea pig very sick and soon die if not treated soon enough( I had a guinea pig once he was my first piggy who died of a respirtory infection sadly) but I have over the years reserched so much on care for guinea pigs and now I know alot about them.
what you should do is make a appointment with the best animal vet in your area.
keep you guinea pig seperate from the other one because the other guinea pig is likely to catch it as well make sure he is comfortable and warm give lots of fresh water and vitiman C (which is the main foods for guinea pigs because they can not make it them selves) watch him and stay close to make sure he is not getting any worse do what the vet will tell you(also I think it would be best to make sure your not sick in case you might of passed it on to him wash your hands before you hold him(just try and remember that

now its to go on now and wait till the best.
Hope I did help and which you best of luck to you and your little guy
Guinea Lynx :: URI
p.s this may help you>
Guinea Lynx :: Emergency Medical Guide .