One of the players in my campaign has near enough all of the source book content unlocked on here where as I just have the physical books. Basically what I want to know is if I buy a Master tier subscription will I and the other players be able to access his unlocked content, or would he need to buy the subscription and invite us to a campaign?

If anyone in a campaign has a master tier sub, players or DM, then all content of all members will be shared with everyone else. The DM (the person who made the campaign) will still have content sharing control, but they don't have to be the one with the subscription

An example I like to use is: player A has the legendary bundle, player B has a master subscription and enables content sharing, player C and the DM have nothing. All players and the DM now have access to all the books. The DM can use content management to restrict access to some books for players B and C (but since player A owns everything, they can't be restricted).

Yes, if you buy a Master tier subscription, you will be able to access his unlocked content, as long as he is in a campaign with you.

Basically, what the Master sub does is it allows the subscriber to enable Content Sharing in a campaign (doesn't matter if the subscriber is a player in it or the DM). When Content Sharing is enabled, all books owned by all campaign participants (players and DM) are shared. The DM then has a toggle to turn off individual books from being shared (for example, the pre-made adventure they are running).