Got a Bite: Should I Keep Fishing?

My son is the author in question here. He wrote and self published his first book in May 2005 (young reader book). Finished a spy novel November 2005, and I have been sending email queries since then. We think we might have a bite from an agent. Asked for the complete manuscript and received it last week. After she received the manuscript she sent me an email with the question, "I need to know whether the ms has been sent to any publishers; if so, to whom. Is it currently with any other agents?"
I replied the truth and said no one has the full manuscript at this time and only one other agency had asked to see the whole manuscript back in December. Should I continue to send out emails? The vast majority of emails have not replied back. Of the roughly 4% I did receive back, it is always the same story; good luck but too busy to take on new clients.

A note right off the bat: it's none of this agent's damn business how many other agents are salivating over the ms. FYI, for the future. If she wanted to ask for an exclusive, which I'm totally opposed to, that's one thing, but she's feeling out her competition by putting you in a position of full disclosure, and I don't like it. Agents assume that you're getting other reads - that's what queriers do - and I don't see why she "needs to know," other than to be nosy.

Ok, whew, with that out of the way, yes, yes, YES, by all MEANS keep submitting! One bite, and I'm sorry to say this, is NOTHING. Do the math: of all of the letters you've sent out, 4% have been interested. Now, what are the odds that this sole agent is going to love the ms so much that she'll want to sign your son? Certainly not 100%. Certainly not even close to 100%. I say this not to be rude but to be realistic. When you're on the agent hunt, you have to get your ms in front of as many readers as possible. I wasn't content unless 5-10 agents had my work, whether as a partial or a full, at any one time. Someone dinged it? Out went another query (or five). Because, the odds were, that only a small percentage - if that - would fall in love with it enough to want to rep it.

Agents have to be absolutely SMITTEN with a ms to offer on it. Don't bank on this one swooning. Not because she might not, but because if she doesn't, you're up a creek without a fishing rod and no bait. Instead, keep the rod in the water, and if she bites it, well, then all the better. Besides, consider this: isn't it better to have two offers than one? The more agents you dangle bait (i.e. queries...I'm getting carried away with this analogy, me thinks) in front of, the more likely it is that you'll land another fish.