If he is a qualified and reputable coach, I would be inclined to listen to him rather than randomers over a forum.
Bench is a crappy exercise anyway. It's good to be able to say "I can bench X" but that's about it. You will see better hypertrophy from DB pressing. You are less likely to pick up an injury through DB pressing.
Just start off with some light DBs, warm up and work your way up through the weights. If you can do X for 12-15 reps, go for about 2.5-5kg more and you'll probably then get 6-10reps. Stick with that weight then.
It is somewhat transferable, but not to a large extent. People usually find that it takes a little while to get used to the difference.