I am 14 and I just changed to trail-running shoes this summer. I made the switch after lightening up to a sub-20lb load a couple years ago. This may not apply to your son like it did to me. But here's how it went for me:
1. Switched to a light load. (Was still using traditional boots)
2. Started carrying trekking poles.
...Let me stop with that for a moment. For some ultralighters (like me), trekking poles are considered to be something that greatly reduces strain on the rest of the body, adds balance and coordination, and make the miles fly. So, I think they are great in conjunction and alongside with boots & trail runners alike.
3. Got tired of the clunky boots, and upgraded to trail runners. They are so much nicer. My feet aren't burdened by the unnecessary weight.
...Note to anyone else out there considering switching to trail runners, attempting to carry a 40lb pack in trail runners requires a level of ankle strength that most people don't get from their 9-5 desk jobs. So, that being said, a switch to trail running shoes should occur after or simultaneously with the lightening of pack weight to less than 25lbs.
Trail running shoes Pros & Cons:
Lighter, and more comfy
You can wear a single pair of trail running socks (which are far comfier than traditional method of a liner and thick wool sock)
Provide less protection
Not suitable for loads above 25-30lbs. without extremely good ankle strength.
Can be expensive.
May not be entirely suitable for extremely trecherous terain.
I encourage your son to make the switch. Like you, I cannot imagine ever getting back into traditional boots in summer. They are such a nice lift.
P.S., popular belief has it that trail runners provide little ankle support. On the contrary, most of the ankle support is in the heel and arch areas of the shoe. If your ankles are so weak that they move sideways too much in unltralight footwear to the point where it is at all uncomfortable, lightweight footwear may not be for you.
Double P.S., I am not a foot expert, and if there is any factual flaw in what I say above, please correct me. Thanks