There are tons of links with ideas on how to help your child swallow a pill with water...just Google it and you'll be amazed on how many links there are on this topic. Many kids can not take pills until they're pretty old...like 10 or older, just because they developmentally and/or psychologically can't do it. And in that case, you may have to get creative if this vitamin is a must for his health.
If it's a gel tab, cut it or poke a hole in it and squeeze the contents into a drink or his food. Some kids do better just chewing the gel tabs up and/or spitting the outter layer out after getting the liquid out.
If it's a solid tab, try crushing them and mixing them with a strong flavored food, or try crushing and mixing them in a drink, or perhaps there is an alternative version of this product on the market you can use.
Many manufacturers of vitamins (especially Omegas/fish oil) that have strong or bitter flavors offer the same products in custard form (so it's like dessert or a pudding) or a liquid form that can be taken in a spoon or hidden in a favorite drink. Many vitamins come in gummy form, so it's more like getting a sour patch kid or chewy candy.