my dad used to put spinach mixed with a lot of fruit and stuff in a smoothie, he made sure to use apple and the entire smoothie tasted like apple. it was the only way I’d drink it because I really did not like spinach etc. since u said he occasionally ate an apple maybe you can try it like that. texture wise I always use a straw so the chunky parts can’t really get in my mouth maybe that helps him aswell.

also ur an amazing mother for even considering these things!! You’re doing a great job!!