You can't force him to build a nest, some males just don't unless in a breeding situation. And even then there are lazy bubblenesters who blow just enough bubbles to hold the eggs. I've even had males who didn't start making the bubblenest until they were already spawning, and just blew bubbles as he collected the eggs.
What is your current set up? Have you tried putting him in the presence of a female to see if that helps?
I just spoon out another males portion of a bubblenest and dip it into where he is supposed to build HIS.
I scooped about a teaspoon of bubbles from haruki's tank and put it under spoofs cup, and over night he covered half the 2 gallon tank with bubbles