Fwb doesn't want to have sex?

Hey everyone I'm quite stumped here and need some good advice! This guy who has been my fwb since thanksgiving just told me he wants to only be friends. The only things we've done are just sexting and talking about meeting up but we never do and I finally asked him why and he said when he gets turned on he wants me and when he's not turned on he just wants to be friends so he decided to not be fwb anymore. To me this sounds fishy and I wish he'd just tell the truth! What do you think? He knows I dont have feelings for him btw we've been strictly just friends. I just have never dealt with a guy who goes from wanting sex with you so bad to just wanting to be friends.