IVF was rare in 2006 and melania was only 36 and there wasn't any height PGS to use for embryo selection back then. But they were rich so they could afford to do stuff that wasn't in the mainstream. I put the IVF odds at ~10%

Multivitamins seem popular in that demo so it seems likely but not causal

HGH or SARMs or something seems likely because he has the means and the motive. Probably improves his odds of getting his D wet by a lot if he is tall and has a six pack.

male height is normally distributed with an SD of 2.5" around the midparent, but hybridization might increase both the SD and the mean slightly. Trump is German/Irish and Melania is Slovenian. 6'9" is approximately in the top 2% of the distribution you'd expect from his parents using a mean of 6'3" and an SD of 3"

(Mother’s height + 4 inches + father’s height)/2=son’s height

Barron’s expected height was 6’3. He’s 6’7. Not a stretch. That’s like a person who was supposed to be 5’10 turning out to be 6’2.

6'9" now, and still growing. He's 18 and Boys can keep growing until mid twenties

how do you plan to resolve this?

If there is credible new report, or admission of it prior to 2040.
Not a perfect system, but ¯\_(ツ)_/¯