he needs to want to eat it himself tho, maybe try let him understand.. and try let him help prepping in the kitchen so he can continue the preps himself.. try diff things, put on music, make it fun, do silly contests

he needs to eat before adding shakes

properly measured fruit/veg mealsmoothies w extra protein powder might also be a great idea.. alone or in addition.. if he's more a liquid type. 

gift him a foodscale, blender and a juicer if u can, w recipe book for smoothies perchance. maybe that'll make it fun for him somehow. 

das my brainstorm