Thankyou for your question.
It is not good for your guinea to be outside when it's really cold. They need to be where there are no cold winds,no drafts and need temperatures that are 65-75 degrees.
If you absolutely cannot bring him in it sounds like you've done the best you can Allison.
Here's more information for you.
If you are from the US see this link:
If you are from UK,please see this link:
Let me know if you have any questions please? Otherwise just click "Accept" and leave a positive feedback.