The best thing to do is to not respond to his meltdowns and not say a word. When my daughter tried a temper tantrum when she was a toddler I just sat down and watched her and I was able to calm myself. She stopped after a few minutes. I don't think that you have to punish temper tantrums. It was good that you didn't give in with the candy bar. (Is this something new that you are doing, because if it is this is probably why his tantrums are getting worse. It is the best thing to do, however, they will get worse until they get better which will happen).

When kids are getting interventions (his therapy) it can get worse before it gets better.

Yeah, I agree, I wouldn't take the "I hate you" personally.