Idk what to do

I haven’t had sex in months and then comes along this generally attractive guy and long story short we wanna sleep together today. Only problem is sometimes during sex I feel bad to say no to things. He always asks if he can finish inside me but I’m on no birth control and today in particular I’m just finishing my fertile window I’m at 8% chance. Even if I want on my fertile window I’m scared to let him and idk what I should do. He said he would be willing to get me plan b and stuff but I’m overweight and I’m pretty sure it wouldn’t work for me. Could I get pregnant since my fertile window is ending or should I just tell him that I really can’t, I wish I could let him but I just feel bad denying him that.