I was the regular babysitter for a little boy with Asperger's until he was old enough for school. He is now 11, and doing very well. He is in regular classes with special enrichments throughout the day, and has a nearly normal social life. My own older brother was "the weird kid" while we were growing up, very smart but socially backward and totally obsessed with all things relating to building; all his difficulties from back then totally fit the Asperger's profile. Had the knowledge been available back then, I am certain that he would have had a much easier time. As it is now, he's quite successful in the construction and design industry, and married with nearly grown kids.
Yes, it is frustrating -- you have a great breakthrough one day, and ten minutes later, it's as if it never happened. My advice is to smile and keep going, and do your crying in private or with a trusted friend. There are many on-line resources available that I hope you are taking advantage of. The more you know, the better. There are also support groups all over the place.
While it is important for you to give as much support as possible, do not fall into the trap of over protecting your son. He still has to learn to cope with the real world, so you must develop a tough enough skin to allow him to fall sometimes! Also, don't let anyone make you feel as if you are failing in any way because your child is not doing exactly the same things as all the other kids -- we all know parents who play the "my kid is better/smarter/prettier than yours" game, and it is particularly hurtful when there is something holding your child back. Don't fall into that trap.
Best of luck. I wish you the best.