He has stated in an email that he will not be paying it. I had set the payment terms to 28 days. This means that I'll have to wait a month to start the claim process, beginning with a letter before action. Since he has already stated he won't pay, can I start the process now?
I am NOT a lawyer.
It is my understanding that in the United States, if you have evidence of an intent to default on payment or a contract, you can start the legal processes of repercussions due to that default, i.e. small claims.
Certainly someone stating, in writing "I won't pay" is evidence that there is direct intent to breach any contract for payment.
There is nothing stopping you from getting in touch with a lawyer now and getting things set up proactively. They will be able to give you better advice that is tailored to these circumstances. Much of the specifics, such as what constitutes reasonable delays and even intent to default (as brought up by others) may be subject to state statutes.
Your lawyer may still elect to not pursue this until the 28 days has lapsed, but at least by that point you'll have done the necessary legwork already.
If you feel that the invoice is worth more than the cost of hiring your legal representation, then go for it.