Welsh cob and a connemara both fillies the highland made about .5 of an inch more but I think that was because she was filling out rather than going up. I think geldings may grow a little more as the act of gelding them makes them less stocky and longer in the leg but I only ever have fillies
I must admit your boy looks very well proportioned so wouldnt expect him to go up much more but I hope I am wrong
Bitter experience tells me if you dont want them to grow they will shoot up 3 hands, if you are desperate for them to grow even a little bit then they will resolutely stay the same height forever more!
I do know of a filly that didn't grow past two. She was out of a big 17.2hh Irish mare out of Primitive Proposal (16.2hh). The filly was 15.2hh at two and never got any bigger. That said, although she didn't make the height her owner wanted for eventing, she was loaned out and seems to have turned into a cracking little horse.
Personally, I have the opposite problem. It's not that my 20 month old is huge, but I was secretly hoping she'd stay on the smaller side. Instead, she's around the same height as her mum in front, and bigger behind. I keep looking at her and thinking she doesn't really seem to be growing anymore (which is fine by me), and then I see her from the back! Now I'm starting to worry that she'll stay ridiculously croup high and conformationally challenged forever, especially after reading some of these replies
Interestingly, my youngster also has Primitive Proposal in his bloodlines on his mother's side. I've come to the conclusion he's finished growing and I'm pressing on with the diet! Any extra growth will be a plus. I bet yours evens out in the next year. Even if he doesn't, it may not be a disaster. My niece had a cracking jumping pony who was 14hh in front and 15hh behind. An absolute rosette machine, never a day's ill health, still going strong at 24.