Acne, no matter where it appears, has nothing to do with any STD.
The only realistic risk here is getting oral gonorrhea. Syphilis is a possibility, but you'd have had to come in contact with a chancre, and you probably would have noticed that on his penis. I wouldn't worry about this in the absence of symptoms like a red/sore throat.
I started to have some back acne though after like 6 months.
I am very athletic and sweat a lot.