It does indeed sound like your son has thrush, which is a yeast infection inside the mouth. Since this has to be treated with antibiotics, you will need to call your doctor and let them know you suspect thrush. If you're not sure, try gently wiping the white spot with a clean burp cloth and see if the spot clears or if it stays. If it stays, it's definitely thrush and needs to be treated by a doctor.
Giving a bottle at bedtime or not isn't the cause of thrush. It's an overgrowth of yeast. My daughter had it when she was about 4 months old and didn't have teeth yet. She was also breastfed. Some people (no matter the age) are more prone to developing yeast infections than others.