My partner has clear stringy stuff from his penis when we have sex and horny. but says he doesn't cum is that precum or cum

Can't be sure, but: I'd think it is pre-cum. Cum is more often not clear, but creamy. Also, trust his experience about this to be better than worse and if you doubt his honesty - don't have sex. Trust is a big thing and good sex is not possible w/out it (IMHO). That said - pre-cum can cause pregnancy too! Not as likely, but definitely possible. Use condoms from the beginning of sex play.