There are so many great ways to get iron on a plant based diet. Most legumes are iron rich and you can make everything from black bean burritos to lentil loaf to daal (a classic Indian dish made with red lentils).
Tomatoes have a lot of iron, so you could make marinara and up the iron content by serving it with chickpea noodles and broccoli.
Some other great, plant sources of iron include...
tahini (use it to make your own hummus - wonderful to dip raw vegetables in)
oats (use to make porridge and add iron and sweetness by serving it with some prunes)
leafy greens (if your son isn’t used to eating these, put them in smoothies with some mango, banana, soya milk, avocado, and flax seeds and you have a delicious, iron rich meal)
tofu and tempeh (marinate it in some low sodium tamari amd maple syrup and then bake until crispy - tastes amazing)
There is a wealth of information on the internet - everything from nutrition information to recipes to cooking tips.
As for fibre, you won’t have any trouble so long as you’re eating a whole foods, plant based diet. All whole plant foods contain fibre, so it’s easy enough to get, so long as all of your calories come from these sources.
Happy eating! It’s so great to hear about children who are choosing this lifestyle for themselves.