I'd also be worried - and you need to be clear with him what behaviour you find acceptable and what is not. school should be doing much more to support you with this - if you've already spoken to them 3 times then its clearly not working.
I've got 2 boys and one could definitly be easily led but knows for sure that this wound not be condoned.
yeah i would be worried about the culture that the school is creating - i think i would talk to teacher put my concerns in writing anti bullying policy lea then change schools and do it all sharpish whilst he is young
oops, I don't know much about 'boy stuff' but this strikes me as rather more advanced than I'd expect from 5-year-olds. Are there some particularly violent kids in ds1's school? I wouldn't like it at all either.