Hi Casso,
The earliest I can find him is aboard 113648, NOVO, January to September of 1915.
He has 4 cards in the Fourth Register of Seamen. His CR10 has his id photo.
It will be difficult to pin down when and where his Dis. A was issued. Probably the only way to find out what ships he served aboard would be to back track his service using known Crew Agreements. You would need to start with NOVO and find out what his previous ship was. We know that in January 1915 his previous ship was 136164, TEANO so you would need to seek out that Crew Agreement for 1914 which is held at the Maritime History Archive at Newfoundland to find his previous ship and so on.
https://1915crewlists.rmg.co.uk/document/192337
https://1915crewlists.rmg.co.uk/document/192333
https://1915crewlists.rmg.co.uk/document/192329
The Dis. A or more formally the Continuous Certificate of Discharge was first introduced in 1900. Prior to that an individual Certificate of Discharge was given to the seaman as he left the ship.
With some more research I can place your grandfather's book as being issued sometime between 1900 and 1910.
Any questions please feel free to ask.
Regards
Hugh