6 year old vegetarian - need advice

[wonka]

Need advice on six year old becoming vegetarian, he's not great with too much dairy really upsets his tummy. We use some soy products and very small amounts of dairy.
He's a good little eater but is very small on the 0.5 percentile.
He's very rational about why he wants to become a vegetarian so I want to let him.
Am I making a mistake how do I ensure he's eating a balanced enough diet and do I need to give him multivitamin? Any meal
Thanks

Thanks all. You've been amazingly helpful.

Here's the fudge recipe and link to where I got it from here

It's totally delicious and pretty nutritious.

Ingredients:

1 cup raw cashew butter (or nut butter of choice)
1/3 cup coconut oil
1/4 cup cocoa powder
1/4 cup pure maple syrup
1 teaspoon vanilla extract - optional

This amount is one jar of Meridian cashew nut butter. Warm slightly in a little saucepan with the coconut oil so they are easy to mix. Add everything else and stir until smooth.

Transfer the mixture to a dish lined with clingfilm, for easy removal. Freeze. Cut into pieces and eat pretty soon out of the freezer else it goes somewhat gooey.

I've made it with ground almonds instead of cocoa powder, and am going to try desiccated coconut next time. Sure you could add chopped nuts and dried fruits if you wanted. There are non-freezer raw vegan fudges out there but this way is much nicer imo.