The only time outs that work with DD are when we sit calmly together and I nurse her and then I explain to her why her behavior is inapropriate.
With DS I tried before 18m to put him in the 'box' (Dutch word for playpen) but this was making him even more angry, so his behavior was getting worse and then he would get more box time and so on. We removed the box and his behavior improved dramatically.
Maybe your son needs some physical outlet. An old matress to jump upon or a trampoline, or something like that.
