We've always done a lot of hiking with our two kids and my son can now do some serious mileage in a day, starting in middle school. Two data points: at age 11, Colorado River via Bright Angel (20 miles RT, 9,000 feet up+down) and at age 12 Half Dome (18 miles RT, 10,000 feet of up+down) both with no complaints nor needing assistance or encouragement (beyond wide-ranging discussions and conversations); BUT, a day hike isn't the same as cranking out 20 miles day after day after day.
My daughter at age 8 also went up Half Dome, nominally unassisted (no one carried her), but Mom carried all her food, water and clothes, held her hand at times, was encouraging, and dispensed "energy pills" (M&Ms) as needed. We were all ready to turn around at any time and had expected to, but she kept plugging along. But 8 isn't 5 and our daughter is amazingly physical with great strength-to-weight (she can do an Iron Cross on the rings).
In my mind, there are two things you HAVE to do:
1) give up any idea of finishing a thru-hike in one summer by doing mega-miles before and after you hike with your son. Just forget it. You can't slow down for 9 weeks and finish in a season (unless you're at an extreme level of fitness and experience).
2) get in a 3-4 night trip with your son as preparation and fine tuning for a 10-day trip before committing to 9 weeks with him on the PCT. A 10-day trip is long enough to learn what mileage you can both enjoy together, your food requirements, and you need to know that to plan food drops, etc - you don't want to arrive at a resupply with 4 days too much nor 4 days too little food!
Until their growth spurt in middle school, our rule was miles=age. i.e. 5 miles backpacking a day for a 5-year-old. We're in this for the long term and feel it is most important to keep it fun so they WANT to go on family trips and have a life-long activity that they enjoy.
At 5, he can take some volume (3-4 pounds of pads/quilts) but essentially none of the weight, so you'll need to be quite UL in your gear and technique because you'll have all the weight.
BPL's own Erin (and hubbie Hig) did 800 wilderness miles this summer with their two kids "at the pace of a 4-year-old" which they figured at 6-8 miles per day. The two-year-old was mostly carried by Erin. Mom & Dad have uber UL and thru-hiking cred having done 4,000 continuous off-trail, human-powered miles from Seattle to the Aleutian Islands hiking and pack rafting. They take their kids everywhere, they live in a yurt off the road system, and the whole family has a high tolerance for dirt, extreme weather, body odors, and cramped quarters. Your son would have the advantage of being one year older and being on a trail, but have heat, more UV, elevation, and slopes to contend with.
Edited to add: I'm glad to see you are planning a 9-day trip with your son as a proof-of-concept. On river-crossings, DO NOT strap anyone to anyone else. There's just too much chance of getting a strap snagged on a rock or submerged log and going under. Maybe cut armholes into his CCF sleeping pad so it can be worn as a vest for warmth and as a life jacket for river crossing ("Daryl Daryl" has posted with such designs). Get him in swim lesson now and through the winter. However much he can swim now, more skill is better. Both for safety on the river crossings and for fun swimming in lakes along the way.