To simply answer your question, I believe the general rule is 15% or more and they can attend YC.

If you are simply adding him as a cofounder so that you are not listed as a single founder, don't do that. It may seem like you have to have a cofounder to get into YC, but founders who are new to working together are a bigger red flag than a single founder, and there were several single founder companies in the last batch.