Feed him a yummy treat when you put on his leash. Work up to making him sit for the treat while you put the leash on. It would take a very stubborn dog to refuse a treat
If the treat doesn't work, smear some peanut butter on a spoon or heck even on the floor and let him lick it while you put the leash on. If it works for cutting nails it should work for attaching a leash