Is it normal for my 6 month old baby to stomach breathe while sleeping at night? He pulls air from his stomach and onto his chest, it looks like wave

Yes. Abdominal breathing is normal for infants; it's actually the form of breathing taught for martial arts, yoga, trained vocalists. When we take "deep, cleansing breaths" we are breathing abdominally.
Sleep position. It is usually recommended to put a baby on its back to sleep. However, when a baby masters rolling over both ways, he/she might feel more comfortable to sleep on the stomach. When this happens, your baby will start preferring that position making it less likely to sleep on the back again. Don't worry. With that new sleeposition, he will learn to keep his airways open to prevent suffocation.
Yes. We breath with our diaphragms, which move our abd. An important observation is does your child feed and grow well. Feeding is the hardest thing your infant does in infancy.