I'm running a game and one of my players built his character as someone who is surrounded by spirits at all times. So Spirits Bard is the perfect one and I really want to let him cast Summon Shadowspawn/Undead with his spell slots. So I made a feat to give him for free which just adds them to his spell list and spells known, it does not cast for free at all. However I tried this feat on a duplicate of his character but it says the spell can be cast At Will. How do I make this use spell slots to cast?

Instead of a feat you would have to make a customized version of their subclass or race and add it there. Then switch them over from their original to your version.