Headaches can deffinitely be caused by bruxism. As can jaw problems. In some adults it can also cause next tension & muscle aches, as all the muscles are connected.
I grind & clench, have done for years, force of habit now I think.
Our teeth sit in a periodontal membrane which acts like a shock absorber, they move microscopically when we eat/grind. This is why grinding, say in children, doesn't tend to do a lot of damage. Also, in children they are still growing, so the membrane that surrounds their teeth isn't as firm as an adults would be. Can be worse in mix dentition children [children with adult & baby teeth] as they 'fight' to seat the adult teeth into bite.
In the case of young children, it can be where the teeth are trying to align themselves into the bite correctly. When teeth come thru they are very ridged & dont sit in the bite correctly, so we grind to get them to sit into the bite. Not much will be done for children. It can be a habit they grow out of once they have all their teeth, but many dentists are reluctant to treat at a young age.
With adults, stress is a major factor. Concider seeing a TMJ [tempromandibular joint] specialist of you have permanent jaw/head/neck/shoulder/upper back ache. Constant tiredness, waking feel lathargic/like you haven't been to sleep.
I haven't heard of smoking making the grinding worse, or affecting the teeth worse. TBH an adult can do plenty of damage to teeth by gringing, with no external factors.
Scampmum, I really reccomend your dh gets a new guard, otherwise he can grind the teeth badly & they may need extensive repair. They can be cleaned normally & I put mine in denture tab 2/3 times a week.