My youngest son had tons of sensory integration issues when he was young. We noticed it early because he was so unhappy all the time. He would cry about clothes, the position he was in in his carseat, the hum of the lights in grocery store...everything made him upset. I truly believe that had we not intervened as early as we did my son would have been autistic.
My advice is to read The Out of Sync Child as soon as you can and contact your public school system for an evaluation for him. If they try to brush you off, keep at it. You have to be an advocate for him. Don't take no for an answer.
What worked amazingly well for us was theraputic listening. Make sure anyone who sets up a program for your son has been educated by Shiela Frick at Vital Links. She has done more research in the field than anyone else. It costs a lot of money if you have to buy the CDs but if you can find a PT or OT who will loan them to you while you're on the program you can save. We started seeing positive changes within days of him listening to the music.
My son also benefited from body brushing. That was also one activity we saw lots of benefit from.
Lastly, I got him to qualify for free preschool through our public school system based on sensory issues alone. Generally they want kids to qualify in two areas but I wouldn't drop it. He is in a specialized classroom with 3 adults and 7 kids. The adults are all trained in dealing with kids with sensory issues. He has an IEP and I get to help set what his goals are for the coming year. However, this began a year and a half after we had already been doing other things.
A good OT (my primary consultant was a PT but that is more unusual) can do an analysis that will let you know exactly what your son likes and dislikes as well as give you a program of play activities to help him.
My son will begin kindergarten next year at a new school. He may still have an IEP with a couple extra things we want to continue to work on, but he should transition into a regular classroom with few problems.
Good luck!