Alice was the same exact way (I was working with her in the house and out in our back yard). I tried treats in front of her, praising when she finally did take a step etc. The only thing that actually got her walking and not being concerned about it was when her shots were all finished and I was able to walk her around the neighborhood. I had to gently pull a bit for a second but once she realized I was going to keep walking and it was going to be fun, she followed like she never had an issue at all and now she's fine with it anywhere. If he's had all his vaccinations (at 18 weeks he most likely has) try a real walk. I know it seems like he has to get used to the leash first, but for Alice, she needed the distraction and fascination with her first "walk" to get over her aversion to the leash and realize what it was for.
Might not work for every dog, but if you haven't yet, just go for it and he may surprize you
