You need to be the one main actor here. Keep him on a leash, and practice "heal" so he is close by you. Then go for a walk on a street you know storm drains are. When you are approaching one, do NOT tense up - dogs pick up on that. If he starts to get scared, redirect his attention to you, and keep walking like it is nothing. It may take many times of doing this, but you can overcome it, just a quick tug to say "focus on me," and ignore the scary drain ... and he should overcome this.