He'll tell you by refusing the formula. This will happen gradually, over the course of several weeks. Normally, a bird will refuse 1 meal, but than want to be hand-fed later on. This might go on for a couple days - he'll ask, you'll make it, but then he'll only eat a tiny bit, and will eat his big bird food instead. Then at some point later, he'll refuse another meal, and you'll go through the same process. You'll offer, he'll only eat a little... it can be a bit frustrating, but it is normal. This whole process can take a while, and you really need to let him set the pace. Keep offering until he no longer asks. There may be times when he's refused a meal for a couple days but then one day he wants it again. Give it when he asks. It can be stressful time for the human!! This is how normal weaning takes place. How many times does he want you to hand-feed him right now? It is essential that you feed him whenever he asks right now, as much as he wants to eat. This is what parent birds would do. And at the same time they will be showing him how they feed themselves and encouraging him to feed himself.
I would assume he's sleeping in his room as long as it's dark and quiet. Personally, I think you should have him with you as soon as you can in the morning. Mine is with me when I make and eat breakfast. She's either "helping" me or she's eating. Birds are flock creatures - they do everything together. When we keep them as pets, we need to try to provide this for them - the flock experience. Is he in his room by himself? Do you have other birds? Is this just for sleeping at night? Do you have an area in your living area that he's in during the day? He needs to be with you as much as possible - when you're home, he's around, either in a cage or on a play stand, or sitting on/with you. He'll nap when he needs to during the day -mine naps a couple times a day, either in her cage or on her perch. Sometimes I put her in her cage to chill out if she's trying to "help" me too much, but I know she's happier when she's with me or the family.