My son (age 15) and I have ridden a couple of thousand miles in the past few years--road riding. We like mountain climbs and descents. He was on a comfort bike until a month ago, when he acquired his big brother's road bike--an older Cannondale Saeco with an aluminum frame. The other day, standing on the pedals and sprinting hard at about 25 mph, he lost control, veered right, and hit a concrete storm-sewer inlet at the side of the road. He crunched the front wheel, bent the frame, and dinged himself up pretty good--thankfully no head injury or internal injuries. This whole time I've been riding a mountain bike with 1.6-inch road tires. For at time I also had a road bike (a nice, older Klein Aura V). To me the road bike felt a little twitchy in its handling, and, with safety in view (as well as my wife's peace of mind), I decided to stick with the mountain bike, which felt a little more stable and of course has stronger wheels and wider tires. Now we need to decide what kind of bike to get for my son. I'm not finding a great deal of discussion on the internet about relative safety of road-style bikes vs. mountain bikes for road riding. I'm sure there's a great deal of variety of handling characteristics within each category, depending on wheelbase, frame stiffness and geometry, wheel diameter and weight, etc. Sure would appreciate any helpful discussion of the safety issues here, or pointers to web sites where these things are knowledgeably discussed.
I would say it's debatable and depends on the individual's preference. Mountain bikes (MTBs) obviously have more traction due to wider and more grippy tyre designs. However a road bike could hold a turn better due to lack of shoulder on the tyre. Speeds tend to be lower on a mountain bike due to added weight and those grippy tyres. On the other hand these slower speeds could be a disadvantage in road traffic. (for reference I ride both an MTB mainly off road and a hybrid with road tyres for paved trails and roads)
I would recommend looking at better/further safety equipment (knee pads, elbow pads, gloves etc) and/or training rather than a new bike. For one, It's cheaper, but it will also install confidence.
Another option is to try with a rented mountain bike to see which your son prefers.