They will flip when they are ready to. You can only help by giving him more tummy time to strengthen the core muscles . Each baby has their own speed and what people say are just general remarks.yih as the mommy know best. Flipping for my baby was around 4mo and sitting assisted was about 5mo. We should not try to force them to achieve milestones too early as that may develop their wrong muscles first. For example a therapist told a mom before she let her baby sit in exersaucer too early and hence the baby did not want to crawl as she is used to straightening her legs in exersaucer
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