We have found immediate intervention is the key to putting the meltdown to rest before it explodes. I was recently dropping of my daughter (5) at day care, and within one minute of arriving, she threw herself on the floor, started banging her head and screaming. It was caused by something small, which I can't even remember now. I picked her up and put her under arm (no small task with a nearly 50 lb. angry kid), carried her back to the car, took her home and called out of work for the morning. I knew if I left her in that condition, the whole day would be hell for everyone--day care, school, our family. She was fine after about 10 minutes of squeezing and rocking. We had a good morning and after I put her on the bus for kindergarten, I went to work. My recommendation would be to remove him to a quiet place with just you and him, where you can pull out your sensory "tool box" and use your tools until he gets back under control. I'm betting you are the only one who truly knows what will work, and although it's nice to have a helper sometimes, in meltdown situations, mom is usually the one who has to address it. When my daughter is with daddy, he is strong enough to just pick her up and hold her really tight until he feels her relax. Then he gives her a snack and they go outside to swing or monkey bars or run for about a half hour. It certainly isn't for the faint of heart, though, and not everyone can call out of work or spend 45 minutes of their day totally focused on calming their child down.
My second recommendation is to read the book Effective Parenting for the Hard-To-Manage Child. It is really helping our family with managing our SPD girl's behavior.
My third recommendation is to investigate dietary changes. We began the Feingold Diet (feingold.org) a month ago, and meltdowns are rare now, and when they do happen, they're over in a couple of minutes. The one mentioned above was a week before we started Feingold.
I hope this helps. You're doing a great job! Only you know in your heart of hearts what will work best for your kiddo.