Could he be teething?
My kids, would reject feedings when they were teething.
A 1 year old... needs to drink milk as you know.
Perhaps ask your Pediatrician.
When he flails himself down in a tantrum... maybe he is trying to convey something else? But at this age its not like they are verbally articulate yet, and their emotions are not even fully developed yet. AND also, what they have in their heads, is not what they necessarily can do, yet. Thus a child gets frustrated.
Also when he flails himself on the ground and gets mad if you "help" him... then just back off. Let him deflate on his own. Kids, WILL deflate on their own... it does not always require "us" to "stop" it. Often times too...a child just needs to get their yah-yah's out... their stress release etc. Just like an adult. At certain times of the day... when my kids were younger, they did that too. Kids go through that. I can almost predict it like clock-work in my kids. So at certain instances, I LET them get it out... because I know it is just their need to spout it out. Then as they get more articulate...TEACH him about "feelings" and the names for it. ie: mad, happy, sad, etc., so that he can then start to TELL you how he is feeling. Sometimes they just need a hug too. Imagine, if you were PMS'ing and stressed... and no one let you express it... you get pent up, right? Frustrated? So, gradually teach him how to express himself... as he gets older, he will understand it more.
When my son was 2... he'd actually tell me "Go away, I want to be by myself..." or, "Me mad... " and I was so proud of him.... because HE knew what HE needed... and I respected that and it taught him how to "navigate" through his feelings.
Start teaching him the words for his emotions... especially for boys, they need to know they can say how they feel. And then do that with him too.
Is he talking yet? Perhaps, teach him sign language as well... so he can convey things to you. Teach him FUNCTIONAL words.... like: hungry, tired, happy, sad, milk, eat, owie/hurt, etc. Functional words are the words that will covey an 'action" in order to get the help they need. Then teach him nouns, so he can say "objects" that he wants.
And maybe he is over-tired? Have you noticed if he does this at certain times? Like when he is tired or over-stimulated, or frustrated or even hungry?
A great book series is: "Your 1 Year Old", which is a book series on each age-set of a child... although written years ago, it is still very pertinent and describes what a child is going through at each age juncture. Its a quick easy read.
Also: "What To Expect: The Toddler Years" is also great.
You can get them both at www.amazon.com
Drinking juice... is not needed. And if anything, dilute it 50-50 with water. Too much sugar in it anyway. My kids didn't drink juice until after 2 years old.
But, I know you said his intake is not optimal. So I would really ask your Pediatrician. Sometimes... with kids who are picky or underweight... "Pediasure" is used. It is a "meal" drink for toddlers...fortified with vitamins.
All the best,
Susan