Hello
I have a 9 month old Staff and he is aggressive towards puppies to the point I can let him of the lead, and he loves a run.
I have had him castrated and have had him since he was 7 weeks old.
H ehas been socialised with dogs as we went to classes and I still go classes now.
He is very dominant even with the cat he lkies to stand over him and chase him!
I dont know what to do? What do you think it could be. I dont want him to harm a puppy, his hair stands up and then he tells them off and snarls I havent left it long enough to get into a fight.

Lots of dogs do not like puppies; if your dog is aggressive to them do not, under any circumstances, put someone else's puppy at risk.