There is a certain amount of reassurance when a partner says he is uninfected; and if he was, the average transmission risk from such an exposure is only around
once for every 500 events.
Still, it's high enough to seriously consider PEP. However, you also should consider contacting your partner and asking him to have another HIV test at this time. If negative, you could safely not take PEP. Even if you have started, it can be stopped once his test is known to be negative.