L.D.
Giving into his pickyness is not the answer. You are only encouraging it. Offer a variety of foods at meals (both likes and dislikes). If he is hungry enough he will eat what you give him. Your job as a parent is to offer healthy foods on a schedule. His job is to choose which foods to eat and how much of each food. You may find Ellyn Satter is a good resource. She has written many books.
To answer your question about V-8 splash. I think the Splash is NOT 100% juice. Which means it has added sugar. The fusion I think is 100% juice. I've tried the Juicy Juice Harvest and it is disgusting. I can't emagin any child liking the combination of carrot juice with fruit juice.
In general offering liquid supplements aren't the best solution. Liquids fill kids quickly and often lowers their appetite. 100% juice still has a lot of natural sugar; often the same as or more than soda. They also aren't learning to explore new foods. If you do choose to go this route I would limit the juice to no more than 6oz a day and I would serve it with meals.
I saw some people suggest pediasure. This is a complete nutrition supplement/replacement. It is really meant for children that are have issues gaining wt. It's not really supposed to be used to combat pickyness. Talk to your doctor before using this type of product.