Hot Rod is pretty much the same way. He is such a mamas boy. We do go to the park any way and he does from time to time go up to other dogs. He does not like any other dog approaching mommy however. He's growled and woofed when that has happened.
One of the parks that we go to has agility equipment and Hot Rod knows how to do that so that is a plus. It is something that he can do and I think he likes that he and mommy have something to do.
I would suggest just keep taking him into the small dog area. I've always been told that you should take the leash off as soon as you go in as it is a sign of heirachy if they are leashed.
Maybe you can find someone there with a smaller dog and plan to meet up regularly. This way they can at least make a friend and maybe it will be just what he needs to open up.
Best of luck to you, we're struggling too.