I bought accounts earlier this year for the same terms. One of the smarter things I've done, as all the accounts were priced correctly to be profitable.
A couple of caveats here, though. It is likely you will lose a percentage of the accounts, even if you keep pricing the same. It just happens anytime there is a change, as you always have those customers who have remained with a certain business because of loyalty, but won't hesitate to switch if the business changes hands. I started out trying to purchase about 18 accounts, if I remember correctly. Four or so told the seller they were going to do something different before the transfer even took place, and another bought their own mower shortly thereafter. I can see this definitely being a factor when dealing with an LCO who is 80 years old, as I am guessing many of his clients have been with him for many years, and probably decided to remain customers as long as he was in business.
One thing you can do is tell him the deal sounds fine, but you will only pay for those accounts who are still customers at the end of the first month. Even if they drop you shortly thereafter, you will have recouped most of your initial investment. Best of luck.
Dependable Mowing Service