I could battle my kids about eating and about being 'picky'... but I don't. But instead, I have found what, at this phase-juncture, they will eat as far as fruits and veggies. We personally eat a great variety of things, from all different cultural foods, and my kids ebb and flow.
My son is SUPER picky...but he will eat artichokes and nectarines.... we never pushed it on him. Or he will eat lettuce. JUST lettuce PLAIN. My daughter will eat a lot of different things & exotic things.... she is just more adventurous and not orally sensitive, which my son is... in different aspects. So, I know this about my kids.... and as such, they do eat healthily and a variety of things... which, at each age juncture, more things gets "liked" into their diet/intake... happily. So that now, per their ages... their "encyclopedia" of foods they eat, has increased.
I don't expect them... to like/eat everything I eat. But I serve and cook whatever I do... and do not cook any differently, just for them.
Luckily my Husband is not a picky eater either... otherwise, that would be more of a problem... him being an adult. But for kids... their palate... always changes.... it is not fixed... so to me, it is not a crises... if at 'this' juncture, they do not eat a buffet of veggies/fruits.... in their repetoire. Because, kids naturally go through different likes/dislikes for foods... and their taste-buds develops...
And, I give my kids multi-vitamins too.
And they are healthy and strong and grow like weeds and developmentally and are in the 97th percentiles for growth... even my "picky" eating son. I personally, do not quibble over what or how much of a veggie or fruit they eat.... because, I know, next month... it will be different... and it changes during growth-spurts too. But my kids are curious about foods/types of foods etc., and to me, that is more important... than actually making it a 'battle' or a power struggle... I do not want my kids to get hang-ups about food... or intake... but rather, to KNOW their bodies.... and their bodily cues for when they are "full."
You can always ask your Pediatrician... for ideas.... they see picky eating kids all the time. Or rather, his Dad/Mom... should be the one's asking their Pediatrician... IF it is a problem. IF the child does not have any weight gain problems or growth problems or developmental problems... then fine.
But if he only 'drinks' foods... and not anything 'solid' that requires chewing.. then maybe his Dad can ask the Ped about that too.. Maybe the boy is sensory sensitive or orally sensitive... or, he is just going through a phase....
You can't 'make' a kid try or eat something. In elementary school, I had a Teacher that FORCED the kids to eat EVERYTHING on their plate. She made kids cry because of it and they couldn't even leave the cafeteria until they ate EVERYTHING and she inspected their plate. She was eventually fired, from her job... after many parent complaints. My parents included.
all the best,
Susan