Hi. When to buy an outfit

Re

So my brother getting married in June. My son in 19 months in 2T clothes right now. When do you think would be a good time to buy the outfit for the wedding? He going to be in the wedding they pick out what they like for him and it’s not to bad price. But I don’t want to end up getting him the wrong size. I also don’t want to wait to long and not get it in time.