The Family Finder test can be good for finding close cousins such as 2nd, 4th etc, however if your son does not know the names of any of his biological relatives, then I don't know how he will be able to work out where they may fit into his family history.
At least with a YDNA test if he happens to get lots of matches with men with a particular surname originating from a particular area, then he at least might have something to work with. Similar with a MTDNA test.
Kimbrey asked if he had his adoption file. Do you know the names of his biological parents? If you do then you
may be able to find a paper trail for them. By all means go ahead with the DNA tests but they are generally best used in conjunction with conventional family history research.
I don't know whereabouts in the world you are, but due to the popularity of DNA testing in the USA you generally get more matches with Americans - it is simply a numbers game. Just something to be aware of.
I'm sure others will have more points to raise and opinions and advice to pass on to you.
Best of luck with the tests - the results are always interesting.