I did the JMT solo in 2008 over 15 days, including 7 from MTR to Whitney portal, no resupply. This year we will be doing a 25 day family trip including kids age 10 and 12. We will resupply at Onion Valley.
I don't think it's quite 15 miles out of the way, round trip. Probably more like 12 (You miss about 2 miles of the official JMT). My plan is to leave the family camped at Kearsarge Lakes for a zero day and slackpack out to the trail head and back. This might work for your son too.
A couple options:
No resupply. I would suggest on a southbound hike of up to 20 days to not resupply south of MTR. Take your time on the northern half of the trail, do a zero at MTR and stuff yourself, then head south with 8 days food. I found the southern half easier than the northern because I was acclimated to elevation and the trip.
Decide on the way. Do as above, but if it looks like you aren't going to make Whitney Portal, make the unplanned resupply at Independence. Hitch into town and then either resupply from the meager options at Valley Market or bus to Lone Pine.
Mt. Williamson Motel "Strider's Resupply" Complete Package ($175 per person, $275 double occupancy) that includes:
One night’s lodging; One hot breakfast; One load of laundry; One pick-up shuttle from Onion Valley Campground Trail Head to motel; One drop-off shuttle from motel to Onion Valley Campground Trail Head: Receiving and holding one resupply in our secure, air-conditioned facility.
I THINK that there is some limited cell service between Kearsarge Pass and Onion Valley. If not you definitely get coverage about 1 mile down the road.