I have found that jumping helps trim up the tummy some, remember you still want them to be deep in the girth, but tucked as well. A good feeding program and a good conditioning program normally gives you a good base to work of off. Your weanling is to young to jump. Jumping is good for adult horses bc it makes the horse use their stomach muscles. They do have stomach sweats but i've never invested in them as i have heard from about 95% of the users that they dont work or do very little. A well known trainer/breeder i know works her weanlings 5 minutes everyday in a roundpen all out, full trot nonstop and her horses look nothing short of amaaaazing. Prob just 5 minutes working each day and trotting over poles (not jumping lol) will help him pick up those legs and stretch those belly muscles and use them.
As for the trotting, there are really two ways i could go upon this, actually a couple
. Remember use voice commands or clicking/kissing ext. Here are some simple ways to go about training him to trot on command.
1) Start off walking, something he knows. He needs to learn to move forward. Carry a dressage whip or crop with you in the left hand and when you ask for him to trot kiss and tap his butt. Keep doing that and then eventually just kiss and only use the crop when he doesnt trot at the kiss. He should learn that kiss means trot and if he doesnt trot he gets a smack on the butt.
2) Another option, agian this would be my second option bc it doesnt teach them to keep moving forward ...it kind of goes offward a little. Well walk and ask for him to trot, when he doesnt yank that lead and make him move back away from you or turn him in tight circles. Walk some more, ask to trot and if he doesnt make him back right then and there and pivot him a few times. Eventually he will learn, she wants me to move and if i dont she is going to make me back and turn. He will want to move forward and learn that there will be concequinces if he doesnt.
3) Have your mom or someone walk behind you w/ a lunge whip or can full of rocks or even just there to clap there hands or something. When you ask them to trot, have the person behind them make some noise to make them move forward. It works
. Eventually take away that person and your on your own.
Also, right now dont do any work on backing yet. You want him to learn to move forward right now. Save backing ext for later
. Dont want to send any mixed signals. Just teach him what that kiss means and what that pressure on his poll from the halter means.
ALSO, remember to reward every little thing. Even if its just something small, make it seem like he is the smartest horse ever and make sure he knows he did that right. He should have a clearly painted black/white picture of what he is doing wrong and what he is doing right.