No, there's no risk to this. Hands are not an efficient way to transmit STDs. There's exposure to air, not enough friction, too many transfers (your hand to his mouth, then to your mouth), etc., etc., to really sustain anything.
The only ways to get an STD are mouth to genital (oral sex), genital to genital (rubbing, grinding, penetration - all while unclothed), genital to anal (rubbing, grinding, penetration - all while unclothed).