What helps me a lot is the book...Baby wise, Preschool Wise and Child Wise. The series goes up to teen years. As a fellow business owner (travel), you may not have time to read the book so below is a summary addressing your topice. However, I highly recommend reading the series.
At this stage, the preschool wise books talk about setting your child up to win. If you know it is near nap time but you gotta take him, make your trip brief. Even in the real world, I recognize my kids schedules and plan for them, be it near a meal time (with snack in the car) or near nap time (whatever I am doing I keep it short) or a group setting (if either of my kids are not feeling group play I respect that and they have to behave, no break downs we just simply cut the trip short. They have gotten the point).
The book also talks about our role in the tantrum. I have to be honest with myself, did i compromise on discipline or making a rule and not follow through earlier in the day. For example, my daughter had the worst break down in the store. But, as I recalled my day I compromised most the day or ignored behavior. So the tantrum manifested at the grocery store. The outburst you see started long before the outburst.
Finally, accroding to the babywise books teaching sharing starts in small settings. Try not to pressure him into sharing or if he feels something is his and it is not, time to remove him from the situation. Yes, that means you are on your toes when you are in a group setting.
However, in a very short time, my kids got the point that I was not going to react to their tantrums and that they would have less play time when they had a tantrum.
If you get a chance get the Babywise, Preschool wise, childwise series. The books have been a life saver.