I would also say this-- be prepared to leave a situation if he begins to feel uncomfortable past his ability to cope. I am finally to the point where, with my service dog, I can leave a situation on my own and wait outside for my family to regroup with me, so I won't force them to miss shows or anything like that if I start to hit my overload point, but I cannot do that without the dog by my side. However, it's important to prepare mentally and realize that you may not get to do everything you want to. Does he have rescue meds? Definitely bring those; i.e. Klonopin, Ativan, Valium, etc. I would also advise taking several breaks throughout the day; if it's possible, take a nap midday because it will help bring his anxiety levels back to their baseline state, and you can enter the parks again with a fresh slate.
I know for me, as someone who is not bipolar but has debilitating anxiety issues, having short breaks throughout the day is a major help. I am at the point where I can speak up and ask my family if we can do a show or a snack because it's a way for me to settle down and, like I said, bring my anxiety levels back down again. If you have shows in mind that you want to see, plan them out strategically throughout the day to seperate long stretches of parks, crowds, and extreme overload.
Again, this is just my suggestion as someone who has anxiety issues myself, and I know that's part of bipolar!