I would also go with an ice cube tied into a wash cloth. If that doesn't work, you've already done what you can and he's just going to have to cope. At some point, when we feel pain that we can't handle it does start to numb on its own. The initial cutting is the worst, so once the actual cutting through the skin of the gum is done he should much better.
We're not always meant to eliminate every ounce of pain and discomfort, by the way. It's okay unless he's nonstop screaming and he can't cope at all. If he's coping, albeit fussy, don't worry so much. These are valuable coping skills he's learning.