I have been during a time period where crowds were non-existant for almost our entire trip EXCEPT EE day at MK! We stayed in the park until about Noon and then it was just too crowded. This was during a very light attendance period and you are going during a time with more average attendance so I'd imagine it would be worse.
If it were me (and we are all different) I would avoid MK on that first day. I found our best experiences in the MK are when we go first thing in the AM and leave around lunchtime. You don't want to have the first day be one of frustration and it wastes time going in, realizing it's just too crowded and heading over to another park. (Been there done that!) I know that would upset my kids more than just heading to another park first.
Take what I say with a grain of salt ... I'm a line hater. I hate lines! I'd much rather work my schedule in a way where I can avoid them! I walk right by if the posted wait is more than 15 - 20. Therefore my experience on EE day afternoon in the MK is that we had to walk by everything! It's also a Sunday so you will have locals as well ... double whammy.
We always find if we get a late start EPCOT is a great place to head to. That afternoon when the MK was crowded .... we headed to EPCOT and it was empty!
Our first family trip we didn't go to MK first and the kids were fine. In fact, many guidebooks advise saving MK for closer to the end so the other parks do not pale in comparison.
Chef Mickeys is right on the monorail with the MK. You can get there from the MK no problem so that is not a concern. It is a great way to start off your trip! Lot's of energy at that place!
I love giving WDW advice but it's based on my own personal preferences. If you don't mind a crowd and you have visions of the first day being one where your son gets to walk into the wonderment of the MK and that is important to you than go for it. But, if you are like me and really don't like crowds. Save it for the next day!
Fear of rides? My DS (5 at the time, he was big too) was scared of TOT because of the preshow (it's twiglight zone-y) ... he didn't mind the fall. That's a trip huh?! His first "real" rollercoaster ride was on that trip and it was the Rock N Rollercoaster!! He rode it twice! I let my son go on whatever he wanted to as long as he was tall enough. I figure if he's not scared why should I make him scared?? The worst thing that could happen is that he does not like the ride and won't want to do it again. Why give him cause to think there is something to be afraid of?
Good luck!
PS - As a side note. Since you are staying at POR save Fantasyland for EE at the MK on Thursday. I promise you will get it all done in that hour! It's amazing and so much better than waiting at other times
WDWO