You cannot change his shoulder angle. He is what he is. His neck is what it is. You can HELP him with working to get him to build his stomach muscles and stretch his top line.
Have a vet come and look at that knee to ascertain the reason for the swelling before even thinking about barrels. He may be fine. OTOH he may have bone chips floating around there that will cause trouble under the stress of barrels. That you really want to find out about NOW, not later. Shame to work your butt off and his training for barrels only to end up with that knee putting a stop on it in competition.
Meanwhile, learn all you can and get a good solid foundation on him. Walking and trotting, trotting up hills, learning to do smooth balanced transitions both in a gait and between gaits both increasing and, especially, decreasing speed. Walk to stop, walk to trot, trot to walk, stand to trot (no walk steps and this only when he is advanced enough). Eventually add the lope. Lope to trot.. smooth and balanced, stand to lope (no walk or trot steps). All this with figure 8's, serpentines, circles (and spirals in and out).. and so forth.
This foundation stuff will take you a year to get going well at this age.
There is something about the outside of a horse that is good for the inside of a man. ~Winston Churchill
) Dinosaur Horse Trainer