Own this movie and watch it 3 times a week, minimum.
Take him to see this movie so it instantly becomes quotable and his favorite and he begs to be a minion for Halloween.
Buy this hat on etsy for $15.
Buy overalls on ebay for $8. (Unless you already own overalls for your toddler, in which case, I'm sorry for your toddler. Listen, it's not personal. I just have a hate/hate relationship with overalls.)
Buy this pin for $3.50 to put on the overalls. Yes, even though you "could" make it yourself out of felt and super glue for like a penny. But this is easier. And you'll swear less.
Buy this shirt for under the overalls.
Put it all together to make an adorable Minion costume that is 50% post-Halloween reusable.
And that, my friends, is how this internet-loving Momma (see also: lazy and crafting-avert) does an easy-peasy Halloween costume for her Minion-loving boy.