A five year old who cannot identify the alphabet is not unusual. This is a skill that takes visual-discrimination and advanced memory skills- both which are introduced repeatedly at the early elementary level. I would only be concerned about this if he cannot recall the alphabet on a consistent basis after a half-year of formal education or continued daily exposure to the concept. If this continues to be a concern, feel free to consult with your school educational assessment committee.
For at home strategies, consider using magnetic letters on your refrigerator, cookies cutters to cut our the alphabet shapes and tracing paper where he can use a marker to go over the lines of each letter.