basic training usually starts of with heeling on leash. Is he used to the leash???
going on or off leash is not the same exercise for the dog; some even say heeling needs special training in different surroundings. i.e.: in the park, in the woods, streets,... .
Did you try just to let him tore back as long as he wants without giving him any possibility of resistance??? so if he pulls back you just go with him, whenever he takes one step forward then you praise him (maybe give him a treat)???
He soon will figure out the right behavior.
If he pulls then stop (let him somehow run into the leash, so that he feels more than just a stop, and then wait until he starts to focus on you, praise him if he starts to look at you; than make a turn right of left, or jaust walk on straight; repeatition is needed quite often; but only the good behavior is praised, bad or not wnated behavior you just ignore (no word, no anger, just behave as written).
have fun figuring out, but it work out fine, usually; and BC are not stupid;
moni
