You will need to wait until he heals completely from the surgery, first of all. THEN you tell the ENT that you need a nasal endoscopy done to see if his structure is compromised. You need to know for a FACT that he doesn't have an occult submucous cleft palate. My son has this and hypernasal speech. With a submucous cleft palate, it is not a good idea to take out the adenoids, especially with a short soft palate, because the adenoids are effectively a "roof" to the palate. When the soft palate is short, it has to work hard to move up to make nasal sounds, and if the roof is gone, it can't reach all the way up. So, you get hypernasality.
I have a feeling there was no nasal endoscopy done before the surgery, which could have told the doctor what your son's structure was like.
If your son's ENT balks at doing this, get another ENT to do it.
I do hope that what I am talking about is not the case with your son. (It is with mine, and I refused to have an adenoidectomy done because I had already researched it.) My son's ENT refused to do ANY diagnostic testing at all and just insisted that I let him do the adenoidectomy, so I found someone else to do testing. THAT'S when I found out my son has an occult submucous cleft palate. My son also had Velopharyngeal Insufficiency - VPI - which is pretty uncommon, secondary to his cleft palate, and most speech pathologists go their entire careers without seeing it.
I do hope for your son that his speech issues are temporary. But if they have not resolved themselves by the time your son is completely healed, do what I suggest. And make sure that if his speech doesn't go back to normal that you get a very experienced speech and language pathologist who has experience with hypernasality. Any speech pathologist who has dealt with VPI will understand the pathology of hypernasality, which is why I mentioned this part of my son's issue, as a frame of reference about the hypernasality.
I know this is all very confusing, but I also hope it will help you. You can show this to the ENT and to the speech pathologist.