If you live above him, I can absolutely assure you he will be hearing far more noise from you, than you will be from him.
I had my upstairs neighbour mention my coughing of a night time. I am asthmatic and my asthma is worse at night. I religiously take my meds but the doctors and I are trying to manage it better. I don't cough all night, usually just as falling asleep and waking up, but I do have a couple episodes in the night too.
That being said, I also hear them hacking up of a night, usually after smoking weed. Mine are more frequent (and as such, I'm sure much more annoying) short coughs, but sounds far less gross.
I also hear their dog barking at 7am. I also hear them shagging. Talking very loudly on the phone at a rapid rate and for ages, at 11pm when I'm trying to sleep. I hear them stomp off to the bathroom in the middle of the night - I hear it because they wake me up in the process.
You're absolutely being unreasonable to even consider mentioning anything. As annoying as it is, he can't do anything about it, and it's probably making him super uncomfortable (I know my coughing does - it annoys me!!). Put some gentle music on if music doesn't bother you. It's what I do when I get fed up of the incessant loud chattering away from upstairs of a night. Or when their dogs doing my tits in.
Bare in mind, if you dare mention anything about the involuntary coughing, he's probably going to come back with all the noises he can hear from you that annoy him. And don't even bother saying you don't make any noise - if you live above someone, they hear you. They can probably hear you when you fart, if you can hear him cough, to be fair. Noise travels worse into properties below, than it does above. So be prepared to hear some truths you may not like. You'd absolutely deserve it as well if you mentioned anything.
Oh, and don't feign concern just so you can bring it up. You're not being as clever and subtle as you think you are, and he'll be right onto it. Being 'concerned' for someone coughing, and even mentioning it, is weird as fuck.