My little girl is 20 months and is still breastfed, albeit, mostly for comfort. I'm also planning to breastfeed her until around 2 years old. She's never had formula. Like others have said, past 1 year babies don't need formula, they don't even need regular milk if they don't like it; it's just a form of calcium and lots of Mums give it in other forms like; yoghurt, cheese etc. The benefit of continuing to breastfeed past 1 is that BM continues to provide the child with the same immunological protection as it did when they were small babies, as well as being able to provide a treasured source of comfort.
You don't need to worry what amount your toddler is drinking at the breast. Trust your toddler that he'll take what he wants and needs. Past 1 I like to think as BM as the supplement to solids so as long as they're eating a reasonable varied diet of meat, fish, veg, fruit, dairy (not necessarily milk) or even if you struggle getting veg down them, as long as they eat fruit; they'll be absolutely fine.