Get straight back to the vet, for starters. Normally with castration, painkillers shouldn't be needed as this isn't an invasive surgery like spaying is. However, it sounds as if this poor young kitten has freaked out over what's been done to him, too young in my opinion. If he's been licking at the wound, chances are they may need restitching (unless surgical glue was all that was used?). In any case it does need to be rechecked. And your vet may give him pain relief, or a light sedation at least to get him through the first few days post-surgery. Or they may even decide to keep him in for a few nights.
You paid for the op, you are entitled to be ok with the result.