I've scanned and emailed documents from my end for him to print out.
I'm afraid that's not going to be good enough for the visa.
All documents must be ORIGINAL (and signed by hand, if required) or the visa will be refused (but you must also include copies of everything or they may not return the originals).
So, you either need to mail all your documents to him in the US (by overnight courier) for him to mail to Sheffield, or he can mail the package to you in the UK (rather than directly to Sheffield), you add your documents and then forward it on to Sheffield for him.
As I said, you only have 14 calendar days from the biometrics to get all your original documents to Sheffield, so you need to get it sorted ASAP.
I think he may have to courier the application to Sheffield in that case.
He should courier it regardless (he should expect to pay between $100-200 each way - he needs to include a return waybill back to the US, to be charged to his online courier account on return of the visa).
They no longer accept FedEx at Sheffield, but he can use UPS or DHL.
I would not use regular mail (Royal Mail or USPS) to ship important documents because they can get stuck in customs for weeks on end and may never arrive. Always pay for a courier service for fast, tracked delivery.
Damn, didn't realise that he shouldn't submit the online application till everything was ready!
His official application date is the date he pays for the visa (which should be the date he submits the online application), so you ideally want to have everything completely ready to go in the mail to Sheffield before you submit it. Then you just have to attend biometrics, add his passport and the printed, stamped biometrics sheet to the package and mail it.
If you want to post a list of the documents that you are including here, we can tell you if anything is missing or needs adjusting. You must meet the document requirements exactly (especially for the financial requirements), or the visa will be refused.