Feel free to ignore this as I am not sure how I would react in your situation. I am so sorry that DS2 needs to take tablets for reflux and you have been unwell.

Maybe ask where he feels sick and see what he says. Then explain that DS2 needs the tablets to drink and is not "sick" per se. Does that make sense? I am not trying to trivialise DS2 condition but if DS1 sees it from another point of view, then he may stop. Is that possible?

Also instead of saying "just a minute love, ds2 is sick, just got to get him settled, just doing his tablet" etc, ask DS1 to get something ready for you two to play when you have finished helping DS2 (if he is into cars, something like "can you get your cars ready on the loungeroom floor and we will play when i have finished helping DS2"). Give him something else to focus on and don't mention tablets so he loses the strong association in his mind.

As I said, feel free to ignore. Good luck.