Hi-
My son is now almost 5 and was diagnosed with SPD around 15 months. He is MUCH MUCH better now, after 3 1/2 years of speech, OT and feeding therapy, as well as us following the sensory diet at home, and getting to know him and what he needs and when. We went through the intolerance to the shoes, the shirts, the pajamas, the mittens, the boots, the socks, etc. We just kept trying until we found something that worked. Most kids are more than one "type" of SPD. My son is MOSTLY a sensory seeker, but also has defensiveness. He is extremely active, and needs a lot of physical activity, which is challenging b/c I work, have MS, and a workaholic husband as well as a 6 yo daughter.
There are some yahoo support groups for SPD; one is "local" to Illinois, and the other is international. I think the local one is SPDshare Illinois, or something similar.
Gluten-free is recommended for autism spectrum, not necessarily SPD. Most autistic kids have SPD as well, but not all SPD kids are autistic. I hope that your son does not have it. I have a colleague who quit work to stay home and do the gluten-free diet with her autistic son and work with him, and she says it's done wonders. I haven't heard of any purely SPD kids benefiting, and neither had the nutritionist my son saw for 2 years.
Please be careful of blaming SPD on vaccines; there are many other factors that have been linked, but no direct causes found yet. In our case, my husband and I both have neurological disorders, and my son shows many characteristics and behaviors that are similar to my husband.
Anyway, I wish you luck. You will be able to best help your son by learning as much as you can about this disorder and your son, and by continuing with a good, sensory trained OT.