Could you ask him more questions about the relationship? How long ago did he meet her, what are her interests, what is the thing he most loves about her, what are his plans for once he gets to California, what is his idea of an ideal relationship....subtle questions if possible to gauge how much he really even knows about her and how serious he is, and what a relationship really means to him. If it sounds serious and valid, you can be relieved; if not, you can hopefully subtly push him in the right direction. The other thing you can do is let him go, but try to get him to promise you that he will call X amount of times per day, get as much contact info as you can - her phone number and address, his itinerary,etc.
I do think that if he could just have some social success, maybe he wouldn't be so bent on chasing this lady to the other coast. And meeting other people on the spectrum through support groups could give him that. But he may or may not be interested in learning about Asperger's and meeting other people with it.
I wish I could offer you something decisive to do. If he does go, just try to prepare him for the possibility that it might not quite work out the way he thinks it will. Tell him that relationships take time and don't always work out; the most important thing you can do, actually, is not to antagonize him so that he is not too embarassed to come home if things fall apart. Make clear to him that you love him and will support him no matter what he does, and that you will help him in any way you can and that he always has a home to come back to. Hopefully, he will spread his wings a little and keep the lines of communication open with you. Get him a cell phone if he doesn't already have one.