One doubles a child's height at age 2 for their adult height. Assuming he only grows an inch in 8 months... That would make him 5 feet tall. If he grows
3-4inches, though (more normal) that's 5'6,
5-6inches (1-2 normal spurts) that just under 6'
7-8inches (a few 3" growth spurts, still normal), that 6'4!!!
So there's a lot in play, still... And 8 months before you'll have a good idea of adult height.
So all doctors can really do at this point is see how healthy he is NOW. As long as he's thriving and developing normally (and in the normal RATIO / percentile for height & weight), they won't be concerned.
If he were lethargic, missing milestones, off ratio (like 90th & 5th percentile a), then you'll see a lot more intervention.
That said... If you're concerned... You don't need to wait 1-2 months to address them with his doctor. Make a well child apt and go in :)