Hello again Shelley and thanks for waiting.
Sorry to hear about Zip's problem, however I have worked out an answer for you, please read it through and see what you think ...
When a guinea pig has an itchy skin condition and there has been hair loss then you would always have to consider mange as the primary diagnosis.
Guinea pig mange is caused by a mite called sarcoptes, the guinea pig form is related to the mite which affects dogs and people [ scabies ] but luckily it is specific to guinea pigs and will not affect other animals including humans.
When I first qualified twenty eight years ago there was no effective treatment for this condition, we used to give affected animals baths in products designed to treat human head lice, sometimes it helped a bit, at other times is was of no use and affected guinea pigs deteriorated and some eventually died. Then we had a breakthrough in the treatment of this disease with the introduction of the drug Ivermectin, this is given as a low dose injection once a week for three consecutive weeks, the success rate is close on 100%.
As this is a prescription drug my advice to you is to have your vet check your guinea pig out, as if it is mange which it almost certainly is then it can easily be treated. Regrettably there is very little you can do to help zip on a home remedy basis.
You can reference this website for further information on the topic : LINK
I hope I have covered your question fully enough but if you would like further clarification or to talk things over a bit more then I will be on-line for the next hour or so and I will be more than pleased to continue working with you.