Should i believe hes not cheating ?

He obviously loves you, and maybe he isn't cheating, maybe he's just being a typical guy who thinks his mother is the one who was supposed to go through his phone and take away his privacy. Ask him out right whether he's seeing someone else and then take it from there. If he really wanted to cheat, he would of! Even if theres temptation there, if he loves you, he'll have the strength to say no.