This depends on your agreement / contract. I will explain what you can do now, after I've said what you should do when making up the agreement.
Ask for partial prepayment. Request half of the amount at start and half of it at the deadline. Split the amount in more parts if it's a large sum. This way, you always have something when the client can't pay (or doesn't want to, for whatever reason) at the deadline.
Put fixed pay-dates in the contract. If you agreed on it, you can make the client pay, even if he can't use the product yet.
Keep in touch. For clients it's hard to get to you if they see problems coming. You can make this easier for them if you have contact anyway. They can easily mention it when there's a conversation going on anyway.
Okay, now what can you do in this specific case?
If you agreed on sending a software packet (i.e. not put the website online), you can just send him it. Send an invoice as well, because you did send him the product. He has two weeks (in the Netherlands that's the period) to pay, that's enough, also when busy with a fusion.
If you agreed on some more services, for which you need the client, you have a problem.
If your contract lists a specific date or period for the deadline, tell him and say you can't help him after that time because you have other jobs to do as well. Subtract something from the price, because you don't give him all the services you agreed on, but make this a symbolic amount. It's his fault, in the end.
If your contract doesn't list a specific deadline, you'll just have to wait. The only thing you can do is ask for a payment of the part you could and did do already, and ask him if you can make an appointment soon.