A few years of medical school and anyone will know something about fractures.
All fractures must be immobilized. A splint is a temporary measure only -- standard of care in today's medical world is to cast all fractures wherever possible to maintain maximum immobility. Like I said, the reason they don't slap one on immediately is because of swelling. As soon as the risk of this goes away, into the cast you go.
If there's "no technology," however, then some sort of immobilizing aid other than a cast might be okay, including a splint, but it would need to be extremely rigid. Maybe something with a metal backplate, with padding on it, and then straps and front metal reinforcement with inner padding. (The inner padding is needed because metal on skin = pressure sores = bad idea. Ask me if whatever design you come up with is practical and makes sense, and I'll review it for you.)
A closed (i.e. not open, no bone showing) fibular shaft (i.e. not involving the knee or ankle) fracture would be the mildest. For a fibular fracture (your shin is the tibia, so that's the other smaller bone), he wouldn't necessarily need a long leg cast and could bend his knee. For a tibial (shin) fracture, a long leg cast is usually required. If you decided that a splint works in better with your design and story, then the splint must go above the knee like a long leg cast would. For a fibular fracture, below the knee is okay. All of these involve the ankle, and the ankle must be fixed at 90 degrees. The knee is immobilized out straight (except for ankle fractures, where it is set at an angle, often 45-60).
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