I was told that if they are having more than 400ml of formula a day, there's not necessarily a reason to also offer a vitamin supplement at that age, since formula is fortified. If he's regularly taking more than that, you probably don't need to. But it is recommended that all toddlers in the UK have a vitamin supplement up to age 5, so if you plan to use one anyway in a few months, I might continue to offer it on occasion so that he continues to be willing to accept it. We started one with our daughter at 10 months as she was sometimes dropping below 400ml a day and we were planning to switch to cow's milk at 12 months anyway. There's probably no harm he'll have too much of any one thing though if you keep him on it.