It's really not a good idea.
Leashes and harnesses are for dogs, in extreme situations, maybe for cats.
Reptiles aren't meant to go for walks. They're ectotherms, cold-blooded creatures, which means wasting energy on useless activities likes walks and exercise sessions is hard on them. Warm-blooded animals have no problem replacing energy. Cold-blooded ones can't do that very efficiently. They tend to move when there's a good reason, not just for the fun of it. That's part of the reason they can sit in one place for hours, while we'd get bored silly and go run off someplace to do something. They can't afford to waste energy like warm-blooded animals can. Long periods of activity (walking for exercise) does them more harm to them than good.
They're not very active by design, it goes hand-in-hand with being an ectotherm. They rest up and save their energy when they're not hunting, protecting or establishing territories, or looking for a mate. Even though they might seem to be asleep, part of the time they're just 'idling in neutral', not doing much, but ready to go if something comes along.
Taking him outside just exposes him to parasites, and disease, it will do more harm than good. You'd most likely take him out in daytime, which is the time he would normally be resting.
The only ones to benefit from reptile leashes are the stores and manufacturers who make money on the sale.