he said he has a bad feeling the baby isnt his? wtf!

so my partner and i have been together nearly 4 years and engaged for almost a year. we have 2 kids already and im 5 weeks. theres never been cheating on either side. anyway, so we were using pull out method and uh it obviously isnt that reliable. anyway, hes been acting super strange for a couple of days and i asked him why and he said "i dont know why but i keep getting this dumb thought in my head that is isnt my kid" and i looked him in the eye and said "youve been off work the last 4 weeks, you know im not like that right " and he said "i know that its just some really weird thought ive had " .... like wtf. he has doubts? what do i do! i never cheated!