What a sweet guy! He looks like he has found himself a lovely home to live in and as you said, he's been bought as a companion so what he can and can't do isn't defined by the love and fun you can have with him all the same.
Like everyone else, I see the odd hocks and have to wonder if he could handle a lot of jumping. He's young and definitely needs some building up, but like Maura said, he should be able to pop over little jumps in moderation happily.
I have to politely disagree with Maura over the statement she made on what sports suit Standardbreds the most however. Perhaps I'm biased, because I have been in the NZ Standardbred scene for over 7yrs now and seen a lot of different types and sizes/shapes of Standardbreds that excel in sports more than carriage/cart pulling. Most horses were specifically bred for farm work or pulling carriages and now excel in ridden pursuits, through breeding and evolution - as Standardbreds have also gone through in the recent years. Whilst they do have conformation faults (like any breed), they can do well in many things, showing, dressage, jumping, eventing and endurance just to name a few. With some exceptions, no Standardbreds will not be able to compete at high competition levels, though can happily take a rider down any path that their willingness and ability allows. I've seen quite a few Standies take out a few well bred hacks, park hacks and even a hunter or two at shows in recent years, even going supreme champion at some well known shows.
I would say not to worry about jumping for now, and work on building his muscles... with such a long back you'll have to work on connecting his back end in with his front... in my experience with Standardbreds, the longer backed horses can run at different tempos front and back. It's not something hard to accomplish, sometimes they just need to be reminded that they're one big horse, and require more leg or a light tap with a dressage whip to get their back end working properly. My gelding was quite long backed, though thankfully was well balanced and didn't disunite. A friend had a similar built horse and he would disunite.. a tap with a dressage whip would help him bring his hind under himself and work at the same tempo. I would suggest also, because he will still be growing into his body to take his training slowly... with being so long, he will need to build his back muscles especially for riding work, and slow and steady will help him immensely.
But to repeat, you did say this sweet man was to be a companion first and foremost. I'm sure you'll get a great bond together where you'll be able to tell what he enjoys and what he doesn't XD.
I hope my opinion doesn't offend :)
♥ Seoul Searchin' for the Lovebug ♥