If he is playing in your world then world progress carries for both.

Warning if you both play on the same PC then he should create his own windows log in to log into steam or cloud data may get corrupted. Probably better to turn off cloud saves if you are going to do that even creating individual windows accounts.

If you are on individual machines it depends on the world seed you are playing on when together. Each world has its own progression that is separate from the character progression. Boss battles are part of the world progression.