@araw, I am not an expert but I'll provide what I would do. Your audiologist should be able to verify the hearing aid by performing Real Ear Measurements (REM). Many audiologists do not do this so you have to find an audiologist that can. Hearing aid styles differ so I'm not sure how much of his aid was already replaced. You refer to it as SEM (inside machine) but it may not include all pieces that could fail. I would assume that the hearing aid is probably okay. Is your son able to determine if sounds fluctuate without his hearing aids in? If he experiences the fluctuations with the hearing aids out, then it would indicate a problem within the ear. He would definitely want to see an Ear specialist (ENT or otologist).
Tony in Michigan