The sprout mix food is safe and healthy so really worth a try and the cuttle bone was created for a calcium boost and keeping beaks short and healthy so you could just sprinkle it into his food. Its a good idea. Try some yoghurt, different flavours, a small spoon full will really boost him, plus its really healthy. Sounds silly but even a little plain vanilla ice cream would help also. Another idea is pure fruit juice. Xander loves most flavours especially apple and pear. Even blend veggies into juice or blend them into fruit juice. All sorts you can try really.
Will he drink milk? Maybe just a little would really help. Just check its safe though for him. Sometimes i mash fruit or veg into Xanders pellets, its how i converted him from seeds and even fruit juice. Its usually not the food but the way its prepared. They are so funny with eating. You will find a method to get the calcium into him, just gotta try everything even if it seems wierd. Keep us updated x