I DIDNT CHEAT!

My boyfriend truly believes i cheated on him with my sons father and i swear on everything that i didnt how can i prove to him that i didnt i dont want this to ruin a good thing he means the world to me and so much more and my son loves him so much i dont want to loose him especially over something i didnt do. Please help me and take this seriously i have never loved someone the way i love this man and i dont know what to do.