Upvote:1
A six month visa would give you six months time. If he received the visa say June 1st then he can travel after June 1st and must leave before November 30th (and if there is a limit for the length of stay, that applies as well).
The unused visa doesn't matter if you apply for a new one. But you got a visa, you paid your fees which are supposed to cover all the cost that the UK had to issue your visa, so there is no reason why you wouldn't get another visa.
It might be that you can't apply for a time that overlaps with your still valid visa. So in my example, if your son wanted to travel November 25th to December 5th, that might be a problem and you should check that.