Gooster suggestion of playing with him across the other side of the field but still in sight of other dogs is a good one. We have used this to help dogs that were nervous or that get over excited by other dogs. When you have his attention on you and playing nicely with you, slowly move closer to the other dogs if you lose his attention at any point then move further away and start again.
Passing another dog going in the other direction sounds like a good Idea, we did some of this with Rye (the one that landed on her head) and I think it helped. Use your tuggie or a tennis ball (which ever is the most attractive to him) to get him to come past the other dog and to you, if he comes straight to you have a good play session with him.
As for doing Flyball, once he knows and is confident in what he is doing (i.e. going down to the box collecting the ball and coming back to you) Then I would gradully get him used to another dog in the other lane. At first wait until the other dog is just coming out of the other lane having completed his/her run before sending your dog to fetch his ball. When he is doing it ok like this try letting him go slightly early each time i.e. when the other dog is half way back up the lane.
These are only my thoughts and some one else might have some better ideas.
let us know how you get on.