How Do I Tell Him Our Son Looks Like My Boyfriend!

I‘m in my mid-twenties and have a young son. I’ve been with my boyfriend since our university days, but I don’t love him any more. I only stay in the relationship because of this child. Around the time I got pregnant, I’d been seeing someone else

for eight weeks but broke off the affair because I thought the kid was my boyfriend’s. Unfortunately, the boy looks nothing like him – he looks like the boy I had a fling with. I don’t want to keep deceiving  my boyfriend about my son, it doesn’t feel right. But how am I going to tell my boyfriend what I’v”e done? People have told me not to tell him but I can’t. I don’t want to have to lie to my son or to anyone. It’s not fair.