That means he's happy and either showing that this is his home and no one else can have it or he's ready to breed.
I do not suggest breeding! You have to know what your doing and research alot, other wise it'll be chaotic and it can probably get expensive.
But it's nothing to worry about! It's a good thing he's making bubble nests. :)
"Life is an adventure full of beautiful things, and a treasure worth more than gold."