My daughter tested into early kindergarten in CO. Our cutoff was Oct. 1 and her birthday was Oct.30. Plus with two older siblings, she was used to older kids and was very mature for her age. She also was a head taller than the other kids in her pre-school class. She was writing her name and spelling out words by the time she was 3 1/2. Her pre-k teachers strongly recommended she start early. All of these factors combined convinced us to let her start kindergarten early (plus I started early myself and never had any problems growing up). Another year in pre-k would have bored her to tears. She's in 2nd now and doing just fine. That being said, if she'd been a boy, there would have been other issues we would have considered. The recent trend has been to hold back boys a year (they call it red-shirting) so even if they make the cutoff, they wait one year and then are the oldest/biggest/supposedly smartest in the class. I'm not sure why parents are doing this, but they are...a lot. So it's possible your boy would one day be a 6th grader (who should be a 5th grader) in a class full of boys who should be in 7th grade. Those years can be brutal, so if your son is the youngest/smallest, etc, life could possibly be kinda tough for him. Just something to consider. On the other hand, my brother was the shortest kid in his class thru middleschool, had his growth spurt and graduated as one of the tallest in his class (he was not held back nor did he start early), so the physical differences in middleschool might not be an issue for your son at all. Also, my son is in 6th right now (although he looks like a highschooler) and I'm constantly surprised at how young some of those 6th grade boys look (I swear, half of them look about the same as my 3rd grader!). I'm not saying keep your son back or start him early, just to make sure you consider all angles. It's not just about how he's doing RIGHT NOW, this decision will affect him for the next 13 years.
Also, re: state of PA, I was told by school officials when we moved here from CO, kindergarten was not required in PA, so even if my daughter had started in another state, when we moved here, she'd have to re-test to start 1st grade early. Needless to say, we delayed our move here for one year so she could just enter 2nd without having to do all that testing again. So even if your son started kindergarten early, it's possible he'd have to 1) repeat kindergarten or 2) repeat testing for early entry into 1st grade...you should check into this. Good Luck!