i completely agree with theama. my boyfriend admitted to me about a lie he had told about a girl he talked to. he said he didnt want to tell me because he knew i would get upset and it was nothing to get upset over. well after that it was like every time i heard that cell phone go off it was "oh my god its her. and hes lying to me about it again". so i just said "ever since you told me that im scared that youre always talking to her and just lying to me about it" and he was very understanding. apologized several times for lying about it (the lie didnt even go a day lol) and has offered up pretty much anything to make me feel better. since he did that i know hes not going to lie about it anymore but i still get those thoughts sometimes. and all i do is think to myself "every time you have thought this was going on you have been proven wrong. so why would this time be any different?" and it helps.

point being, he got that lying to save my feelings is not worth it and stopped doing it. and if he is sweet like you say then he will understand and help you through it.