Your son would be a lot better off if he tried applying directly. Most firms immediately post any openings on their corporate homepages. It's not like they are crazy about paying 30% commissions to headhunters.
Here are some examples of sites he might want to check on a regular basis:
Bloomberg http://about.bloomberg.com/careers_find.html
Morgan Stanley http://careers.peopleclick.com/careerscp/client_ms/external/search.do?functionName=getSearchCriteria
UBS https://jobs.ubs.com/1033/ASP/TG/cim_home.asp
Thomson Reuters https://toc.taleo.net/careersection/2/moresearch.ftl
GE https://xjobs.brassring.com/1033/ASP/TG/cim_home.asp
Barclays http://www10.i-grasp.com/fe/tpl_BarCap01.asp?newms=se
Or, look at a direct link aggregator site like this one http://jobbind.com/japan/list