Yes, it is an ongoing service but in some cases it may not be. Let me explain.
Many people when talking about SEO mix boundaries and are really talking about a broad online marketing plan that may include Social Media, ads and activities to engage users. Why do they mix it? Because one of the factors that affect Search Engine Optimization is the number of visitors your site has as well as backlinks (that could come from your FB or twitter account as well). Marketing is an ongoing service and will always be.
Now, if we stick to SEO itself in its most basic form (that is, the things you can directly control in the site such as coding or article writing, outside backlinks or external factors)... per page SEO may not be an ongoing service as pages should not be constantly changed.... that's why I said that in some cases it may not be.
You can write an article and try to rank it for two, three keywords and code clean HTML and CSS to rank it higher... would you change that article often? No, unless the article is a whole disaster. Why? Because even if it didn't accomplish its goal to rank for a particular term, regular visitors would be confused in the order of the words was changed, or any part of the article itself. And a website gets traffic not for only one article but for a bunch of information. So getting obsessed by one particular page is useless.
There may be a case that the page did accomplish the goal and the Search Engines changed their ranking algorythm and you may have to work again on that particular page. But, again, it may confuse visitors and may not be worth the effort.
The ongoing part would be writing more information, creating new pages, each of them properly optimized.
You talk about goals, not about rankings and do be aware that you may have to use CPC campaings to get that goal. You can't base only in organic traffic.
A goal could be having 5,000 visitors a day. Is it realistic? Well, that would depend on the traffic they already have.
Or maybe that 20-30 people sign up for a newsletter every week.
Or maybe a rise in sales of 10% (this is something that he will understand for sure )
You will go for something that you can accomplish and demonstrate. A number 1 ranking is great but... how long will it stay there? 1 day? 1 month? 1 year? You never know.