Do it now. I had to fire someone two days before Christmas and while it is very difficult it has to be done. Treat the person with dignity and respect, focus on the facts and if at all possible they have vacation or something coming that will ease the burden.

First make sure you are vetting your advisor as much as he/she is vetting you. I would have a personal conversation with other teams that the advisor has worked with in the past. Get to a level of granularity on what worked and what did not. You don't disclose terms here which I appreciate but the percentages are important so I just want to make sure you aren't giving away too much. By "alot of weight in the industry" do you mean that this person will make introductions for you to key customers, suppliers? Or are they well known and you think adding them as an advisor makes you look credible. If you are adding an advisor make it count. You need to identify three to four areas where you are not confident in your teams skills/knowledge about your business model and make sure that advisors added are addressing those very specific needs. If this person is really good then you would want to spent a couple of hours with them every month. If you don't then you should ask yourself why you are taking on an advisor? If you need a more detailed discussion let me know.