I will dissent from the previous opinions. Although I agree that the intent should not be a bidding war, most offers are not open ended for time, they come with a specific time frame in which you have to act. If the offer is from a school that is high on the list, then I would absolutely let other schools that were high on the list know that Junior had received an offer.
Recruiting is weighing one opportunity against another, and this holds true for coaches also. Coaches who are waiting on other recruits decisions before they offer you, are counting on you being there to make the offer to. If they sense that they could lose you to another offer just because they were distracted in other directions, that may refocus them on their true desire, or lack of desire, to recruit Junior. You'll may get a sense of where you stand.
I don't know of a coach that would take it the wrong way, they want to be informed and you'd hate to hear, "man, if we would have known, we would have offered too!"
I am not an advocate of either divulging offer amounts, or using offers from schools not truly in Juniors top tier to do this with though. If the coach were to ask what the offer is, a response of "worthy of consideration", seems most appropriate.
It's a high wire balancing act at times, but isn't every negotiation in life!
Wishing you good luck and good fortune.