I usually use 1 lemon together with 2 spoon of honey and 500 ml of water. I note that as I increase the water content, the juice does not taste as good but I would like to reduce the acidity of my lemon. So, are there other ingredients that I can add such that the taste of my homemade honey lemon juice will not be affected?
You can neutralize the acidity of your drink by adding a half teaspoon of baking soda, but don't do this. Apart from fizzing up like a volcano, your lemon drink, or what is left of it, will taste pretty awful.
What you want to do is reduce the perceived acidity. This can be done simply by adding more honey. I suggest adding a teaspoon at a time until it tastes about right. I like about double the amount of honey you specified if we're talking teaspoons.
You neutralize acid by adding a base. Generally bases are bitter tasting. Black tea is a base, as is baking soda (sodium bicarbonate). The problem is that when you add an acid to a base you get salts. What we call salt is NaCL, which is just one example of a salt, there are many others, and those salts can add all sorts of undesirable flavor combinations. So adding a base to neutralize the acid can backfire badly.
The best way to deal with the acid is either to cover it up, or not add it in the first place. You could add more sweetener as @ChrisSteinbach suggests, or you could add less lemon juice plus some grated lemon zest and maybe a little water. Lemon zest is the very outside of the peel, the really yellow stuff, not the white fluffy stuff. The zest of the lemon has a huge amount of lemon flavor with little acid content as it comes from the oils stored in the peel. I recommend a microplane grater to get the zest very fine.
The zest of lemon/lime is often more desirable in cooking than the juice. In drinks that call for a twist of lemon or lime it's the oil released from the peel in the squeeze that adds much of the flavor.