Clients can be self destructive: "You can lead a horse to water, but you can't make him drink." -- Famous farming wisdom
Sometimes you just need to find another horse.
Another famous bit of "on-the-farm" wisdom: "Never change horses in the middle of the stream."
So, don't give up hope on your client. As others have suggested above, the right way to convince him is to show him that the traffic you're already getting is not buying enough.
Teach him about how to CONVERT traffic into SALES.
And, if you don't know how to do that yourself, which is certainly possible, since you're probably and primarily an
SEO engineer, not a sales engineer, then you have to spend some time on teaching yourself, then concentrate on him.
The traffic you already have sounds like it is very good, considering that you say you've created 170+ sites for this guy.
Think about this:
If you have 10,000 people a day coming to your 170+ sites, who buy only 3% of the time, that is 300 sales.
Let's say we want to DOUBLE sales to 600/day.
Which is easier, to get yet ANOTHER 10,000 people a day to come into your sites, or to sell more to another 3% of people who are already there?
I can tell you from 30 years of experience in both real-world major media advertising (radio, TV, newspapers, outdoor, direct-mail and magazines) this: For a client who has already saturated or nearly saturated a market (as you may have already done), it is ALWAYS easier to increase profits from selling more to existing customers than it is to try to increase substantially the customer traffic itself.
Assuming that you've done at least a fair job of
SEO across the board on all those sites, which is probably true, your best bet is to work on what we call "conversions".
Simply put -- salesmanship!
Present your customers with compelling reasons to buy what you're selling. Dramatize these reasons. Do product comparisons. Do price surveys. Do charts showing main reasons to buy. Analyze things for your customer. Show large, detailed photos that illustrate how your products work. Use extreme closeups. List customer testimonials about how great your stuff or service is. If you don't have them, call exisiting buyers and get some. Get permission to quote them. Produce FAQ pages listing features and common questions about your products.
All the fancy technology using email blasters, newsletter trick-nology and the rest can't fix a deficit in salesmanship.
Sell, sell, sell! Convert the visitor into a buyer. Think of your sites as you would if they were large stores... with thousands of customers coming in the front door(s), wondering the isles and then 97% or more of them wondering back out the door again. These people may have been "just shopping" or information gathering. But, how hard is it to convince only 1 or 2% more of them to buy?
Not very hard.
But, it can be nearly impossible to get another 95-100% more people to come in the door in the first place, which might have the same effect as converting only 1-2% MORE of the people you've already got.
Focus on selling to the visitor while they are already in the store or sites. That's where your money is.
Okay, it doesn't hurt to add a newsletter or email here and there. But none of that is as powerful for profit-raising as selling each person while they are already there, looking at your sites.
That's the way to sell. You'll find out you can double, triple and even quadruple your existing sales, even though you don't increase traffic a bit.
Good luck!