An electronic tuner is essential. But since he already has one, you don't need this feature in a metronome.
Since he's playing bass, he might want to practice with different rhythms instead of just a constant click, click, click... So you might want to look at the
Boss DB-30 (Dr. Beat Metronome), which is $39.99 at musiciansfriend. This model has built-in rhythm
patterns and beat variations, including odd time signatures and Latin rhythms.
(Can you tell I taught at a guitar store for 6 years?)