Yes, the 10-15 min heads up thing works. All three of my boys can be like that. So I warn them before its time to change the schedule of our day. Try it, and see how it effects him.
What helped a little was giving my daughter about a 15 min. or 5 min. heads up if I had to get her off the TV or go somewhere or have her come in from playing. This gave her time to adjust to the idea and reduced the horrendous reaction that usually ensued.
Well, these things are never easy. My middle daughter was the same way, even as a baby. Since she started so young, I knew she was "wired" differently than my other two kids. What a tantrum she could throw!!!!!!!!! I actually had to put her in the bathroom with permanent fixtures so she couldn't break anything. She would get "FURIOUS". Her tantrums could last up to three hours sometimes. It was scary for me as a parent. I too thought "Is she mental?". Sometimes I would have to hold her until she calmed down. She would either be calm and happy or throwing tantrums, nothing in between. For several months, I took her off of gluten to see if it cleared up other problems, and WOW! what an improvement in her personality. Things that used to set her off so easily and quickly didn't seem to bother her anymore. She was the kid with the dark circles under her eyes (allergy shiners). I read that many kids with behavioral issues suffer from cererbral allergies - makes them feel irritable, or spacey, and unable to handle normal everyday stressors. I also don't let my kids get away with much, but this kid didn't respond to normal discipliine. Unfortunately, she didn't want to stick with the diet (she was a very picky eater and still is). To this day, I ask her to consider going back on it (she's 14 now). She still gets very irritable and has a tendency to lash out - she then feels bad for lashing out and apologizes. Food intolerances can lead to many vitamin and nutrient deficiencies and inflammation in the brain and body. I tell you this so you will consider food as a possible culprit in his behavior. I'm wondering if you should consider keeping him home another year until you figure out what is going on with him. A bad first experience in school can continue on for years. The kid gets pegged by the teachers as "watch out for this one" and they pass this information on to the other teachers. The kid might end up dreading school. The kids might distance themselves from him. Just a thought. It's possible also that he would behave better in school too, but you would be taking a chance. Hope this helps you. I remember feeling all alone with this same problem, afraid to tell the doctors for fear they would stick her on medications to control her behavior. Pay attention to everything he does, is his skin pale, allergy shiners, red ears, poor diet, etc.
Good luck!