Let me answer your question with a question: If your son has an accident using your vehicle, do you want your insurance policy to defend him and pay any legal judgment against him for injuries or property damage he causes? Of course you do. And because it will cover your son when he drives your truck, your insurance company is entitled to some kind of charge.
Yes, you should tell your insurance company about your son. However, since most of his driving risk is likely to involve his mother's vehicles, many insurers won't charge you anything for him, provided that you supply:
- A copy of his mother's insurance documents showing him as a "rated driver" listed on her policy.
- A copy of his driver's license showing his mother's address as his primary address.
You asked, "Why would his occasional use differ from any friend of mine using my vehicle?" It would not differ. Your son does not, for example, have any rights to coverage for driving other cars, at least not under your policy. That means if he were to offer to drive friends home in their cars because they've had too much to drink, and if he then caused an accident with serious injuries, only his mother's policy would cover him. Your policy wouldn't extend coverage to your friend in a similar situation.
I do have one tip for you. Because your son has regular access to your truck, his mom's auto insurance won't provide him with coverage when he drives it -- only your policy will. That could be a problem if the teen is involved in a serious accident with potentially expensive liability, because he would have only your liability limits available.
There's an inexpensive fix that his mother can add to her policy called extended nonowned automobile liability coverage. The cost typically is about $20 a year but varies by company. Her policy would then cover your son using your truck once your coverage runs out.
I hope this is helpful to you.