First they should probably be assessed by a healthcare professional.
In my own circumstances, I found drinks like boost or ensure are generally well received. Alternatively, have you tried to use any straws with them? Sometimes that offers increased control to take some fluids in. You might want to look into something like a safe straw if they are unable to suck and maintain that suction when using a regular straw.
www.agingcare.com/questions/get-mother-to-drink-more-water-186514.htm
In my own circumstances, I found drinks like boost or ensure are generally well received. Alternatively, have you tried to use any straws with them? Sometimes that offers increased control to take some fluids in. You might want to look into something like a safe straw if they are unable to suck and maintain that suction when using a regular straw.
Does he cough after drinking liquids?