Tell him to look up:
The failure rate of phalloplasty.
The effects of putting young women into menopause (which is what transition does).
Is he aware of the risks of brittle bones? Sterility? Loss of sexual function as an adult?
Is he aware there are no serious long term studies about outcomes for trans teens on current treatment regimes - this is experimental.
Is he aware of the Cass report
cass.independent-review.uk/publications/interim-report/
...or that several senior clinicians and other senior staff have left GIDS and the Tavistock over their concerns over how children have been treated? (google Dr David Bell, Kirsty Entwhistle, Marcus Evans, and also Keira Bell's court case which was supported by ex-GIDS nurse Sue Evans).
Is he aware that 80% of children no longer feel trans after puberty, but if you give them blockers, 98% go on to cross sex hormones.
He needs to do some serious research, and not from trans advocacy groups but serious sources like NICE and the Cass review.
And calling you TERF? He can fuck right off.