This is as much a problem with adults as children. Think of your work situation. You start the year, your boss (implicitly or explicitly) says "Do a good job this year and you will get a raise". Twelve months later, you're told you didn't do a good enough job and don't get a raise.
Alternately, your boss has a list of goals, and periodically discusses with you your progress towards those goals. Every six weeks you sit down and see where you are. By the end of the period, it's not a surprise whether you get that raise or not.
Which one would you prefer? Pretty much everyone prefers the latter, and your child should also. Obvious, visual representations of the goals and progress towards them are key. Star charts are a very good option; each time he does something towards the goal, he gets a star, and he knows how many stars are needed to get to the reward. Color meters are good for behavior issues; start the day on green, positive things move it up (to more-green?) while negative things move it down (towards red). "If you end the day on green, you can watch TV. If you end on red, you can't. On yellow, you get half the normal TV time."
Further, use SMART goals (this is another business-world term): Specific, Measurable, Achievable, Relevant, Time-Bound. Make sure your goals are specific (not "behave at the store", but specific elements like "Stay within arms reach of me at the store, keep your voice to a normal level, and do not grab anything off the shelf we didn't ask you to take".) Make sure they are measurable - either clear binaries (do not do X specific thing) or countable limits ("Have fewer than 3 accidents"). Achievable - things he actually could accomplish, specific to his personality. Relevant - as much as possible the goal should be somehow related to the reward ("Eat at least 1/2 of your dinner and you can have dessert"). Time-bound: the goal should be limited in time and explicitly stated.
That's pretty important for kids, because they need to be able to achieve their goals most of the time. If you're setting too difficult goals (either expecting perfection when perhaps 95% performance would do, or goals that are just too complicated or inappropriate to their personality) they will fail repeatedly, and will start thinking that they are failures and/or are going to fail before they've tried - and thus won't try.
Keep SMART goals and visual representations and he will have a much easier time understanding - and will over time learn some understanding of long-term goal to reward relationships as well. Expecting a child to understand that relationship without explicit progress signals is a mistake, as it's not something adults do well with either.