I assume perhaps a neuropsych might also diagnose, but occupational therapy is the only treatment form I know of (aside from just trying to deal with it on your own at home). If you look for an OT, you definitely want to find one that deals only with kids and is familiar with SPD (most of them are). The sensory diet is really what you are looking to develop: figuring out his triggers and coming up with activities that sooth them. There are also lots of products designed to help SPD kids, depending on their specific needs. The OT usually has these things for you to try out and see if it helps.

I can't recall if you said what your DS's IEP is for, but SPD is often discovered along with other issues, but can be found on its own. Often it is the kids who don't have any other obvious issues that get missed. And like Kate said, kids can have both avoiding and seeking behaviors and the pattern of problematic stimuli is fairly unique to each person. But the therapy is very helpful, and I also found that for me, recognizing the sensory issues really helped me, especially with my DS's seeking behaviors and occasional meltdowns. If I imagine how the world must look, sound, smell and feel to him and imagine what his body is telling him to do about it, it made alot of his behaviors alot more comprehensible. And it helped me figure out what he really needs to help him figure out how to deal with and control it. For example, in kindergarten his reward for having a day of keeping his hands to himself (sensory seeking) was a few minutes playing basketball with the PE teacher at the end of the day. I now realize that this should have been reversed. Playing basketball actually feeds his sensory seeking needs and therefore should have been the remedy or preventive measure rather than the reward, if that makes sense.

I also think there is some overlap between SPD and the sensory overexcitabilities associated with giftedness.

Here are a couple of sites that were helpful to me when I was trying to figure this out. HTH!

www.sinetwork.org
www.sensory-processing-disorder.com