Our family has recently acquired a Austrailian Sheapherd as our newist family member. The problem I have is keeping a collar on him when you put a leash on him. Recently, i've done reading and will try a halti. The second part is when you apply any resistance on the leash to get him to come or walk he immediately begins to flop around violently like he is having a seizure. Do I just let him flop until he learns that's they way it's going to be or should I take another approach.
First make sure his collar is not too loose. If you can put 2 fingers snuggle around his neck it is good.
I don't think your Aussie is not used to be leashed. My border collie mix was just like what you described when first adopted him at the age of 1- 1.5 yrs old. He was fostered by a farm family for 2 mos before we adopted him. He hated to be leashed and used to lunge, and try to get the collar off.
However, he got used to it pretty quickly. Since that was the only way he could go out. Dogs will learn how far (little) they can go in their daily routine
What I would recommend is to put him on a leash even inside the house for 10 minutes everyday. Every time you leash him, associate it with something good. The easiest thing is to give him a little treat, or give him some training that also involves some treats. Pretty soon he will learn that being leashed is not a bad thing.
I would use a short leash when taking him for walk because it is much easier to control him, and harder for him to flop around.