Lars,
I can empathize. My first "long" trip was only a month but I was 15 and my parents made (rightfully) me go with an older person. Since I wasn't murdered, robbed, and/or raped, my parents let me go on bigger trips.
When I was 17 and the summer before my Senior year in high school, I wanted to do the TransAm. My parents decided I had to have others to go with me. I arranged through BikeCentennial (ACA's predecessor) to ride with two others. We took off and in less than 3 days, I knew we were not compatible as one (the oldest @ 25) had misrepresented himself. I am a meat eater, prefer to ride in the morning and be done by 2pm-3pm, and wanted/needed to ride about 55 miles per day on average so I could be back to school in time in August. Unfortunately, the oldest was a vegetarian, like to ride in the afternoon until dark, and didn't want to be tied to a schedule/deadline.
After 10 days, I convinced my mom to let me try it myself. I finished the trip, again without being murdered, robbed, and/or raped, in Lewes, Delaware (I went through DC instead). Best trip of my life. Mind you, when I say best, I have 50k+ miles of loaded touring and 36 years experience. About 1/3 if this was during summer bike rides before I graduated college and started life. Heck, my Dad is 92 and he still somewhat worries about me.
To you parents I would say: If Lars (and the other rider) is truly responsible and mature (at least for his age), he should be fine. Thousands ride the TransAm every years. The route is well-established route. Locals tend to definitely look out after the young, female (not being sexist, just real), and retired. When I was under 27, I could get help most anywhere, i.e. police would tell me where to camp, look after me while camping, etc. Between 30 & 50 (I am 50), the help decreases then increases like an inverted bell curve with 45 being the worst. Perhaps the locals think what is a fully grown man riding a bicycle and wanting to camp. He must be a bum. The retired are thought of as "s/he is just enjoying something. Great for them". But I digress. in today's era of cell phones, it is pretty difficult to be without communication. Combined with a phone card, make Lars call home every night when possible. NOTE: Some nights it just won't be possible due to no cell service and no pay phones available. But Lars will tell you when he is going out of range, right Lars!. Finally, to alleive Mom's fears, Lars will carry a satellite tracker so you can track him live (every 10 minutes). There are two brands, SPOT and InReach. SPOT is cheaper but InReach is more reliable. The InReach allows you to text each other also anywhere in the world. If Lars is NOT responsible and mature for his age, make him wait or go with at least one older (40+) person as they will probably be too tired to get into much trouble.
Lars will occasionally ride with other riders on the TransAm. Not everyday, but not unusual.
Yes, Lars could get hit by a car and die. However, that is unlikely. Especially, if Lars rides conservatively, uses a mirror, and wears a helmet. The TransAm is old enough so that bikes are expected on certain roads so the locals know to be on the look out.
As an FYI, my daughter and son both have taken solo bicycle trips under age 20. My son (just turned 21), rode from Jasper, Alberta, to La Paz, Baja California, Mexico, by himself this past late summer/fall. He was not murdered, robbed, and/or raped. He was greeted most days with "that is awesome" comments, met the Governor?? of Baja South Department (state), etc. Yes, I worry about them like any parent should. But I also know the pluses outweigh the risks of trip. My wife, a cyclist also, definitely worries when any family member tours, and thus requires me to use the tracker. But she also knows the pluses outweigh the risks.
Parents, go over to CrazyGuyOnaBike.com and review the 1000s of journals about the TransAm. Plan on meeting him in a couple of places, i.e. the Yellowstone or near a relative who lives near to the route.
This truly can be a transformational experience for Lars. He will come back a changed person. Definitely more fit, but with a quiet sense of accomplishment and appreciation of America and its people. As I get older, the trips, while meaningful, do not impact me as much. Perhaps this is due to my extensive travels or with age but regardless, the TransAm is by far my favorite trip. Should you, or Lars, wish to contact me, feel free.
Best wishes, John