My husband would like to build a pool house with a toilet for our pool, but we are on a septic tank and he is not sure how that will work. He isn't quite sure how he would add the new toilet to the septic, any advice ?

No problem! Hubby needs to first establish the sewer line from the house to the septic tank. He can the pick a spot closest to or more handy to the proposed pool house. After pulling a permit he can then cut in a wye or combination wye and eighth eighth and connect to the pool house. As long as you're building from scratch why not add a shower base to the underground rough in and an lavatory on the stack that you will run out the roof. Now your pool house's complete.
Sound like a plan? Cheers and good luck on your project. BTom

Agreed but the key part to Tom's answer is you must pull a permit. This may or may not even be allowed in your locale. Best bet would be to get 3 contractors to bid the rough in and see what they jave to say, Then your hubby can cecide if he can turn this into a DIY project or not.