How do you reckon practicing rolling a barre is going to help him hold down a barre?

It takes a while to perfect this, your finger is just not designed to do this until the muscles are developed. Just keep playing! Use bar chords frequently, they will get better. And as said before, use the side of your finger, not the flat.

Ya, this. When I barre it's almost like I'm barring with the very bony, side of the finger. Also, really working with your thumb to pull the finger tight against the neck can help.

Practice. That is all.

Yeah just sit down 10 mins every day and play some barre chords. Then it comes all by it self. And don't even try to sweep pick. Proplerly the hardest technique to master.

ive been trying to arpeggiate barre chords but the G string and sometimes A don't ring well because usually my second or third finger is infront of the first pinning down a different fret.

Try a Gripmaster. I use one, seems to help not only with guitar but possibly with finger strength for drums too.

Also practice trill exercises with:

Index/Middle Finger
Index/Ring Finger
Index/Pinky Finger

For me that sort of goes Easiest to Hardest...

Also I think there were some good ideas in Troy Stetina's "Speed Mechanics for Lead Guitar" on Hal Leonard Music. Check it out, it's a GREAT book series, short and sweet, everything you need, TAB/Standard Notation, affordable.