[callmebad]

Because you're in your 20s, I don't advocate that you take Viagra.
Can be you're a bit tense.
Because you're doing with WLs and facing them, you either have to stand or don't stand, which can be quite stressful, not like doing with gfs, can take own sweet time.
When I had problem of standing, usually after a few minutes of cock-sucking by the girl, my cock will come up again.
Suggest you try this.
If not, maybe you have to use cock ring or vibrator
You bring the vibrator over the area between the anus and your balls, which is quite sensitive, see if this can help you.
Or, you try to masturbate yourself to see if you can get back the 'fucking' feeling, don't pressure yourself too much if it takes a bit long, the main thing is to get back that feeling to allow your 'brother' to stand up again.
I think yours is more of psychological problems, so don't need drugs or Viagra to overcome this.
Hope what I said here can help you.