Talia yes he probably could have told the horse to knock jason off, but the horse probably wouldnt of listened, because he is loyal to jason. like those fire-breathing horses on that farm (in the battle of the labrynth). percy tried talking to them, but they wouldn't listen, even though percy can talk to them (if that makes any sense)
Samiksha i think that the eidolon inside percy didn't want percy telling jason's horse to knock it off. Also i dont think that percy can talk to horses made from lightning. dont forget, percy can only talk to natural horses like arion, hazel's horse. i dont think percy can talk to jason's horse, tempest. maybe jason's horse was a gift to him from zeus, his dad..