It's a delicate balance between just flicking the neighbour away and saying you won't do a thing (as many here have advised) and finding a way to live in peace, and if possible on good terms, with the neighbours. OP, you don't sound like the kind of person who just can't and won't be bothered with caring about that neighbourly relationship... which is, after all, why you even started this thread.
If I remember correctly, you said that you're in your 20's, so while you physically probably hear very well, you're in a life phase in which noise doesn't tend to bother one as much as it does twenty or thirty years down the line. That's one of the reasons that old[er] toppies who can still hear well don't actually want to go to loud discos and concerts as much, or can't bear to stay there for as long. One's sensitivity to noise changes as one ages. If your neighbour is a few decades older than you, that could explain why the aircon doesn't bother you at all, but sounds noisy to him.
In any case the fact is that - at least from your neighbour's subjective perspective - you are bothering him, and he feels this quite apart from any reality or measure of how loud the sound of the aircons actually is. Therefore, if you're going to achieve anything towards fixing the problem, you will need to reach him on this basis.
It's a good idea to measure the actual level of noise of the aircons, and to find out whether or not they are within the legal limits. If not, go back to the people who installed them and ask them to fix that. And even if the noise is within the permitted limit, you could still ask the suppliers for all their advice about dampening the sound emissions.
The tips to put up a sound-dampening barrier, or sound absorbing screen or trellis are very good and some version of them is probably fairly easy to put into practice. Also consider a hedge or other dense, leafy shrubbery that will break some of the sound.
Most importantly, tell your neighbour the truth: that you yourself can barely hear the aircons and would never have thought of them as loud, yet you do hear him that this is an issue for him. Tell him you're not really sure how much you're going to be able to reduce the sounds to a level that will not impinge at all, but that you are undertaking some things, and trying to find out about options.
Say that you have... for example... spoken to the installers/manufacturers and obtained a tip to do x and y (small actions you can quickly do before speaking to him, such as checking that there is no loose, rattling grill or cover, or something along those lines), and that you will be finding out about possibilities for a screen next to each device. All along, make it clear that you're investigating, and not yet making promises that you will carry out all of the options, since of course you have limited resouces in terms of time and money. Tell him that you're getting advice on a trellis with plants or a hedge within your property, to break the sound. Ask him what he'd think about his perhaps raising the fence between the properties (but first find out the municipal by-law about that... so you know how high he is and is not allowed to build).