Boyfriend still texts ex in a flirty way, what shall i say?? help please?

ive been with my boyfriend for 6months now, but 2months before we got together he was seeing another girl. he quite often looks through my texts and doesnt like me texting other boys even if its just about work, he also questions when i put 3kisses to a text as he says why put 3 if you wouldnt give them 3in real life. i was fine with that and could see where he was coming from and it kind of gave me the same point of view, so he was the only person i would put 3kisses to. however i dont go through his phone like he does with mine as he would say i dont trust him, etc or ask why i wanted to. when he was deleting messeges i saw quite a few texts to the girl he was seeing before me, all with 3kisses even if the messege she had sent didnt have any, or only had one. he also never texts me first and when i text him he doesnt seem very enthusiastic with his replys. i know if it was him in my shoes he would definitly say something but i really dont want an argument, should i be worrying, should i say something, or should i just forget about it? if i say something, what do i say without him saying im being nosey or not trusting him etc. ive seen texts from her on more then one occsasion now and its started to bug me. thanks for the help i really do love him and dont want to cause any trouble in our relationship x