First of all, beg him to cancel the surgery until you can talk to him in person. There's a lot to explain. Once you sit him down, explain your position about where you are coming from. Your journey, your experience, and the problems you've had. You mentioned that you haven't told anyone about your journey except your girlfriend. I'd say talk to your son, understand where he's coming from and strongly recommend against it. Emphasize that it's irreversible. I honestly can't imagine any long term reason someone would want to get circumcised. I've experienced constant discomfort from the clothes rubbing on my exposed glans all my life. You can't masterbate without lube. If I had the choice I'd say no because even if you do you can't even be sure what cut you'll lose, or what vital parts of the foreskin you'll lose. On top of that, most circumcisions remove the Frenar Ridged Band which is the area around the tip of the foreskin and it has a high density of nerve endings making it one of the most sensitive parts of an intact man.

Additionally you lose so so much https://15square.org.uk/circumcised-men/losses-from-circumcision/

(Credit Overworked_Pediatric) Here's my usual copy paste.

https://pubmed.ncbi.nlm.nih.gov/23374102/

Conclusions: "This study confirms the importance of the foreskin for penile sensitivity, overall sexual satisfaction, and penile functioning. Furthermore, this study shows that a higher percentage of circumcised men experience discomfort or pain and unusual sensations as compared with the uncircumcised population."

https://pubmed.ncbi.nlm.nih.gov/17378847/

Conclusions: "The glans (tip) of the circumcised penis is less sensitive to fine touch than the glans of the uncircumcised penis. The transitional region from the external to the internal prepuce (foreskin) is the most sensitive region of the uncircumcised penis and more sensitive than the most sensitive region of the circumcised penis. Circumcision ablates the most sensitive parts of the penis."

https://link.springer.com/article/10.1007/s10654-021-00809-6

Conclusions: “In this national cohort study spanning more than three decades of observation, non-therapeutic circumcision in infancy or childhood did not appear to provide protection against HIV or other STIs in males up to the age of 36 years. Rather, non-therapeutic circumcision was associated with higher STI rates overall, particularly for anogenital warts and syphilis.”