Pulling is an annoying habit that a dog gets into. It sucks for the handler. Both of mine pulled like Freight trains when I first got them. Tika was the worst, she'd pull your arm out of the socket.
Dogs pull because they've learned to pull, and all to often we reinforce the behavior unknowingly, and let them "get away with it".
Often when they pull, they are pulling towards the destination we are taking them anyway, and when they do get away with it ,the behavior is rewarded unintentionally.
Catan needs some serious leash work. It's time to re-enforce the walk at heel. Get him heeling properly. Every time Catan pulls, do a 180 and take him the opposite direction and bring him to heel. If he slips his collar, put him in a harness. If you stop and he continues to pull, become a tree. Don't move, don't yank him back, let him pull until he can't breathe if that's what it takes. When the pressure is off the leash, bring him back to heel and continue in the same direction. If he pulls while you're moving off do a 180 and go the opposite direction.
Don't yank the leash when you do a 180, just turn and walk off. Keep a 3' loop in your hand and when you do the 180 drop that loop. When he hits the end of it, he'll know. Don't use a choke chain, or a head collar when doing this. Use a nice 1" safety collar. You're not trying to put him in pain, you're trying to make him put himself in discomfort.
When you work him at the heel keep that coil of leash in your hand. Every time you change direction, drop that loop and bring him back to heel. Collect the loop back up and do it again, and again and again until he is paying attention to your left knee. Where that knee goes, he goes, or he pays the price. This has to be enforced every time you have him on a leash, by everyone that handles him.
Once again though, don't be harsh with him. Don't yank or jerk the leash in frustration. You want your movements to be smooth, methodical and non emotional. It's training time.
it's sounds involved and difficult, but in reality it really isn't. It would take only a couple of 1/2 hour sessions to turn him around.
Leash work is hard. it's boring and it's generally not much fun for the dog, or handler, but it's really necessary.