Depends on his start in life a lot. I got Jemma as 20 months old. She was 13 - 13'1 the most (she was in bad situation, btw). Tiny... Now (she's little over 4) she's 14'3'' (and that's what all earlier string measurements shown she should be with full growth). No one believed she can pick up the height, but with good food, turn out and care she certainly did (and I didn't gave her supplements, just 12% grain with corn oil and hay, some grazing). Same with my paint - came as 20 months old as 13'2 - 13'3 now she's 15'1 and under 4 years old (still growing though).
Probably an old wives tale but I always heard the cannon bone will give an estimate of finished height. At around 18 months the cannon bone will be at its finished length. At that time if its 15" the horse will finish at 15hh
Yeah, I know someone who's horse had to be put down when it ran into a barbed wire fence......didn't see the fence and ran into it. Speared by a T-post. Two different storied as to why, all but one person says the belgian chased him or her into the fence, but the one person thinks it was a cougar. The belgian is real pushy, but there have been cougar sightings around there.
Very cute, he will most likely fill out: He's kinda in (or before?) the gangly teenager stages, I'd say. My dogs both did that.
As for keeping him a stallion, why not? I've heard of stallions kept for performance.
Plus, another reason - maybe you want to keep him a stallion just for the heck of it!!