I have been "out of the game" far too long to offer current advice as to how things are done, but I can say that if your son desires to be any type of "Criminal Investigator" versus "Federal Officer" (investigations versus patrol) he will need either a Bachelors Degree (at minimum) or expect to serve at least some significant time in uniform not doing investigations or undercover to get the requisite experience to fall under a "training agreement" to get GS-1811 designation.
You still need that Bachelors Degree to get direct hire (no other job experience) for any 1811 position, and can still get to that 1811 via other GS Series after gaining experience. The CG (as a Boarding officer or L/E Officer) is really only relevant if you want a "anti smuggling" job on the water or involving things happening on the water. Both CBP and DEA have marine assets. Army CID, USCG Investigative Service, and Air Force Special Investigations (NCIS has no military members that I am aware of) are more likely to help with a Criminal Investigative job (1811-Series, FBI, DEA, HSI, US Marshall, etc) since they all meet the job parameters of "Criminal Investigator" (1811 Series) where being an ME. BM, MK, OS, Regular Commissioned Officer, or other Maritime L/E related experience is VERY narrow in the scheme of Investigations. Generally, CG L/E people are NOT allowed to do investigations, only patrol or using given intel, but CG experience is good for patrol type L/E jobs. Army CID, CGIS, and AF Special Investigations all have limited "Warrant or Commissioned Officer" positions that work with the civilian GS-1811's and are considered "Criminal Investigators" and trained accordingly.
If you want to be an investigator with one of the "alphabet" agencies, better to get that Bachelors Degree and do direct hire with the agency he desires. If he wants to serve first, and for a limited time, IMHO he would be well served by enlisting in the CG, no other service is going to give him the L/E background the CG will.