As a point of comparison, I will show you what Ancestry gives me and my husband in those categories.
22 Shared Ancestor Hints
6 Starred matches
305 4th cousins or closer
23 Shared Ancestor Hints
0 Starred matches
228 4th cousins or closer
We are both American, so that may skew the number of matches that we received as I believe Ancestry's database currently has more Americans than anyone else.
The starred matches are ones that I made a "favorite", and the shared ancestor hints are based on matches between my and the DNA match's family tree. So, since your son is not a subscriber and doesn't have a tree connected to his DNA results, it makes sense that he has zero there.
The 4th cousins mention he got may refer to "4th cousins or closer". Those matches are based on DNA that he has in common with those people, so they are not useless hints. By looking at their profile and/or tree or contacting them, you may be able to find where the connection is. At this point, it's a matter of getting to look at the results.