From an old article in Guitar Player, I believe he said it was just the "good old two finger pluck", plenty of videos showing him do just that. Supposedly he occasionally used a pick live and on recordings but i can't remember a video of him uing one. His bss lines were always solid and functionl, which they had to be to anchor behind Eddie's guitar craziness and Dave's sloppy singing style, but what was most impressive about him was his ability to play in time and on target and sing those high harmonies. By all accounts a nice guy, and Eddie and Alex treated him like dirt.