I recently bought a guinea pig from my local petco and had to return him due to him having a major upper respiratory problem. I got him back on friday but now he nibbles every time I pick him up. He'll nibble every piece of skin he can get a hold of. I've bred pigs before and never had this issue. How do I break him of this habit. Also he is extremely skinny from the upper respiratory problem. What can I give him that will help him get back to a healthy weight?

In order to eliminate sickness, you need to strengthen her immune system back to normal. Guinea pigs love fresh lettuce every day(NOT iceberg lettuce - it's actually poisonous to them), watermelon skin, carrots: they are full of vitamin C and A. Also, get him a granola bar for guineas from the pet store; it's full of minerals and they need it. Make sure she is not cold or hot, because guinea pigs are very sensitive to temperature change. They will also get minerals and vitamins from hay or alfalfa; it's important that they have all the above for optimum health. pets question and answers,www.5d2d.com
Also, guinea pigs get infections easily when they are not cleaned out regularly.

Feed him lots of veggies!! Carrots everyday twice a day, he can have up to 3 carrots a day until he is a healthy weight again, then only 1 per day. And give him lots of romaine lettuce and green bell pepper.
The nibbling is because he is extremely nervous around humans, probably because he is so young and has the URI so was terrified all week by humans, like you! So he's nervous. Spend lots of time with him and hand feed him often and he will come around. My pigs do that at nail cutting time. Constant nibbling all over my hands, so I understand.