If your son is under 21, then the money fomr a life insurance policy would go towards the things that you would normally pay for him - general living expenses, driving lessons, university, that sort of thing. If you have enough savings to cover all his needs, or he's old enough to look after himself, then you don't need life insurance.
In my case, even though I don't work at the moment, our DD is only 1, so if I suddenly died DH would have to find money for childcare, plus deal with never having an income from me (I plan to go back to work when DD is in school, maybe sooner). So for us it's well worth it. I'm astonished to say the least that an agent quoted you £700,000, especially as it wouldn't have to cover a mortgage. Sounds like he would get a fat commission to me. There are plenty of cheaper ways of getting sufficient cover.