The naked truth is, in fact, the only path here. You said he's examining his feelings, now's his chance to actually do so in more depth, and to learn that you will be honest with him, which will help do away with one potential source of fear. On the other hand, if he finds out on his own it will make out harder for him to accept this, more easier, and could even permanently damage the trust between you two.
Be honest. Now, while it's still a new, mostly innocent flirtation. Give him the chance to be as open minded as he's trying to be.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.