Well, STD's can be transmitted by skin to skin contact (herpes, syphilis, genital warts, and trichomoniaisis) or by fluid exchange across a mucous membrane (chlamydia, gonorrhea, HIV (blood and body fluids).
As you said, that you have touched his penis shaft while helping him in wearing condom, chances of getting any STD are very slim.
Good to hear that oral sex, was performed with condom, so you would not get STD from this act.
Condom prevents, by making a barrier in preventing fluids, semen and blood from transferring.
As such only touching shaft once, during helping him in wearing condom would not transfer any STD.
The high risk comes with having unprotected intercourse -- vaginal, anal, or oral -- with a person who has a sexually transmitted disease.
In your case, from my clinical experience and from studies, chances of getting STD are very slim.
I hope this helps.
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