I have fairly long dreadlocks. My job is at a desk but I have done my share of moving, carpentry, welding etc outside of my job. He just needs to keep his dreadlocks tied back, covered or whatever to keep them out of the way and from getting snagged. They are no more or less dangerous than anyone else with long hair - except they get caught on velcro a little easier.
there are couple of situation where long hair would be funny, not for long though - in a machine shop or inside moving
tank with main caliber gun auto-stabilizing targeting system on - even untied headphone wires would on several occassions pull the head in
Tons of people with long hair work in machine shops. In fact, it's possible that more people with long hair work in machine shops than other professions, statistically, because of the socioeconomic signifiers of long hair on caucasian men (there are far more long-haired white men than there are rastas and sikhs).