play every note on every fret with only your pinky. then do 1 fret slides, forward and reverse, on every fret with your pinky. finally, do a hammer on to every note on every fret with your pinky.
every other day, for 2-3 weeks.
it might help to buy one of these:
Planet Waves Varigrip (note - that's an affiliate link and i make like 45 cents if you buy it through there
)
i have one on my desk and have gotten a ton of use out of it. even then, my pinky is still weak when compared to my other fingers. Also, remember that using hammer-ons with sweeps is somewhat advanced anyways, so be patient and it will come.