How do I lead train 6 wk old puppy?

My pup is 6 wks old and I have the age old problem that when I put him on the lead and start walking, he digs his heels in, despite positive encouragement and treats and the lead slips over his ear. I refuse to have it any tighter. I've only done it twice for a few mins and don't want to persist as I want to do it right. Thankfully he's an intelligent dog (Jack Russel X Chihuahua) I had him toliet trained in a matter of hours, you tell him something firmly 3 times, like 'No' to biting the sofa and he has it! He just cries when I try to take him for a walk on the lead despite all the positive encouragement. Should I just persist? Altho, if I do, he'll slip the lead completely and I don't want him to know he can do that. Any hints or proven methods for clever dogs? Please don't come on here with negative comments as I've had dogs before and 6 wks is not too early to train them.