My neighbor has the same swallowing issue regarding finding the sweet spot of liquid being not too thick and not too thin. She added some hot chocolate to her coffee, to thicken it up a bit, since, like the dad in question, her coffee was too thin. Even though the hot chocolate is dissolved in the coffee, it still seemed to thicken it up just a bit, enough where she was able to swallow it easily and it didn't "come back up." They even have dairy-free hot chocolate now, if dairy is an issue for him. Hope this helps.