pebbles in a jar is a definite no no!!!
work out what he is barking at!
1. if he is alerting you to something - say thankyou and give him a treat.
2. if he is scared (barking, ears back, tail between legs) remove him to calm him down.
3. if he is nervous (bark will probably be accompanied by a small jump) - reassure him and ask for "quiet" - reward for compliance.
without seeing the dog in action it is difficult to give advice but these small tricks might help you.
1. when he is laying nicely and quiet - reward and say "nice quiet".
2. play lots of games to tire him out.
3. keep him busy with chews, toys and time in his crate (he will feel more secure there)