Helping my dad buy his first can, and it's already at the dealer, he did his eForm account and SS account and did his photo, fingerprints, etc. What happens next? When does he pay for his stamp? We are on a trust together. Should get be getting a DocuSign email in a few days? It's been awhile since I've done this so I'm out of the loop.

If things haven't changed in the last year,

We bought our cans through SS and paid for the stamps at the time of can purchase.
Set up our accounts at SS and at ATF eForms site.
SS sent us the DocuSigns to digitally sign for us both (both on the trust) finger prints for both and pictures for both.
They did pretty well with letting us know at what stage everything was at during the process on the SS Dashboard under the Settlors (head of the trust) account.  
But wasn't more then a week between stages and emails.
Double chack spam folder, maybe they sent an email already?

Hope that was worth 2 cents.

Hey HTFA!
All responsible persons should get the DocuSign email, double check in Spam/Junk folders. After the DocuSigns are complete the eForm will be prepared and will then be ready to certify. You all, and the dealer, will coordinate a time to log into the eForms portal and certify the submission together, at this point the tax stamp is applied and the form is sent to the ATF for processing. - Chase