Hi Pogo, congratulations on your new Boxer Apollo. You didn't mention if Apollo is a puppy or adult (probably doesn't matter) but I'm glad you want him well behaved on leash.
I happen to be sitting a Pit Bull puppy this weekend, who has absolutely NO training whatsoever. She's not a bad puppy, but she also freezes when the leash gets clipped on, and I don't know exactly
why but, no matter.
Equipment needed:
puppy
leash/flat buckle collar
long handled wooden spoon
squeeze cheese (from a can) or peanut butter
#1) Apollo needs to learn that having his leash on means something fun/nice/exciting is likely to happen next. In the house somewhere quiet where you two can spend 5 minutes together. Load the spoon with the Cheese/PB, and let Apollo lick the spoon *while* you clip the lead (no cheating and clipping the lead first

). This gives Apollo the chance to associate the sound of the leash being put on with something as nice as cheese (with me so far?)
#2) Now take the spoon and
slowly move it from Apollo a few inches. Does he move at all? Does he at least act like he wants to? Good! Progress! This is the slowest (most boring) part of the whole process for you, but the hardest for him so take it easy on him. Continue to practice this untill he is taking a step or two.
#3) By now, Apollo can hopefully follow the spoon, maybe 5-6 steps, and you two can doodle aimlessly around the house with the leash dragging behind.
#4) Pick up the leash and be prepared to repeat steps 1, 2 & 3, but with you holding the leash.
#5) Go ahead and try it outside, (someplace quiet outside) and again repeat steps 1-3.
This *could* take several days to move from steps 3-4 and from steps 4-5, so plan on two weeks of this 2-3 times per day, 3-5 minutes tops. All you want to accomplish is a positive association from the clip noise of the leash to the fun you are both about to have.
If you have a more toy motivated dog, use a toy to replace the food/spoon. Sometimes, as with the case of the Pit puppy I'm watching now, Maya just doesn't even know that her actions can result in ANY type of reward, so she isn't driven toward one reward or another.
Good Luck and Happy Training!