Tom,
Based upon my own experience, I would recommend that your son continue on a path to become a licensed professional engineer in civil engineering. If he wants credentials in welding, I would recommend that he get certification as a CWI rather than a CWEng. To do that he will, of course, have to have the qualifying experience set forth in paragraph 5.5 of AWS B5.1 - Specification for the Qualification of Welding Inspectors. The necessary experiance is not trivial, but he should be able to collect that experience if he seeks it out while working as a civil engineer for a few years.
The P.E. license establishes a person as an "engineer" in the eyes of the law. That is important if your son should ever want to open his own engineering company in the future or if he would want a senior position with a consulting engineering firm. There is a path to the P.E. via the Ohio principles and practice exam in welding engineering, but I don't recommend it if you can more easily get a P.E. license in some other discipline. If you want to take the P.E. exam in welding engineering, the Ohio board will put a few obstacles in your path if you don't have Welding Engineering degree from the Ohio State University. You can overcome those obstacles, but at the end of the day you may wonder why you bothered to jump through all their hoops. The Ohio P.E. license does not specify in which field of engineering the license holder is qualified to practice.
The AWS CWEng credential may have value someday, but at present it is rare (there are only about 30 CWEngs worldwide) and not widely in demand. I got my CWEng after passing Ohio's welding engineering exam. The welding industry understands the CWI credential (there about 20,00 active CWIs worldwide). I got my CWI credential after recognizing that very few people in the welding industry knew what the CWEng was.
Hope this helps explain your sons options,
Ed Fritsch, P.E., CWEng., CWI