Me and my bro is dueling right now and i attack, then he activates shadow spell. I say that since he activates it when i attacks the attack still goes through since shadow speel does not say it negates the attack of the monster, he says that my monstr cant attack even though he activates it when i attack but not when i summoned it. what do u think

Theres a huge difference when a card says NEGATE the attack (but im not going to get into it), anyways the attack WILL NOT GO THROUGH..... its a simple response to an attack, (ie. Ring of Destruction/Sakuretsu etc. doen't state a negate an attack either, but the'll stop the attack by destroying the monster), Shadow Spell works the same way, it just stops the monster making it unable to attack..... hope this helps

it depends on when during the atk he activated it.

if he activates it in the battle step (same timing as mirror force or magic cylander) then the atk won't go through, since the monster cannot atk anymore.

however if he activates it in the damage step (same timing as injection lily and kinetic soldier) the atk will go through.

this is because since shadow spell directly modifies the monsters atk value, it can be used in the damage step. but once you are in the damage step, it is too late to stop the atk, and so the atk will go through.