I might be out of line here - I imagine you are far more experienced and knowledgable than me, but here's something that struck me:

I reassure him, offer him cuddles and try to soothe him till he calms down. I offer him things that I suspect he wanted and were the problem to see if he just can't verbalise his wanting those things and that's why he's upset.
I wonder if this positive attention is reinforcing the behaviour? Personally, I find Natty's tantrums subside a lot quicker if I just sit near her and let her be until it starts to wind down, then quickly offer a bright distraction. This is hard when a child is hurting themself, so I understand how this situation would be different if your DS is hitting his head against things...
I know how it is to have a kid who seems to fly off the handle moreso than other kids. Natty used to have the occasional meltdown at playgroup. It is always much easier to get through it at home!