An invite for an unofficial visit at your son's age is significant, and demonstrates real interest. My experience is that there are several tiers of visits, with an unoffical visit demonstrating fairly serious interest, and a school's Junior Day reflecting medium interest. The official visit really only comes into play if you haven't verballed over the summer and are still being courted by a number of schools. An offical visit usually implies an offer will be made, but not always.
On the grades issues discussed, my motto is to not let your grades limit your opportunities. the better his grades, the more schools there will be willing to look at your son.