This.
The number of times people post on here with minuscule fibs from their partners - only telling half the story and being unclear as to why they felt the need to question in the first place - and get dozens of people who know bugger-all the situation egging them to leave their partner or question him or go through his phone is insane.
Recently there was a thread on here where a woman had told her boyfriend she was having a shower before leaving his house after him. She actually hadn’t had a shower, she’d just gone home after a quick wash.
He then realises the shower hadn’t been used and repeatedly questioned her and accused her of having an affair because if she’d lied about that, what else was she lying about, etc.
Her reason for lying was actually that he’d told her she was gross and embarrassed previously for not having a shower one night, so she felt awkward and told a silly fib as a result to try to avoid an inquisition.
Correctly, she was informed that her boyfriend was controlling and paranoid and needed to get a grip.
For all we know, this OP’s partner might be having an affair, we have no idea. But I think it’s at least equally - probably more - likely that the OP questions him for details of every woman he ever speaks to and then causes arguments about them if he mentions them in passing again in future with “Oh, LILY, is it, she’s the pretty one I saw you with that time, why are you mentioning her, why did you sit next to PRETTY LILY at the Christmas do instead of one of the older women? Oh so they’ve put you on the same team as Lily have they, why?” etc and consequently he just tried to deflect that by making up a name instead.