Are you talking about an IP PIN or are you talking about the 5 digits it asks you to make up at the end of the return.
If it is the 5 digits you make up, then just enter any 5 digits that you make up.
If it is the IPPIN, he may be able to make a call to the IRS to get it or retrieve it online. If he cannot make a phone call but you can have him sign a IRS POA, then you would be able to obtain the IPPIN on his behalf. If he set one up, you cannot file online without it, you would have to file a paper return.