S.S.
Hi M., my oldest son struggled with the same thing at first. My girlfriend teaches Kindergarden (she has also taught First and Second grades) She said that is pretty common especially for boys to get confused (even until age 8). She suggested trying a visual (with his hands)...I'll explain the best I can.
tell him to think of "bed"
hold both hands approximately arms length away from body,
make a fist with each hand leaving the thumb out and pointing up toward the ceiling,
This will be a visual of a bed (headboard and footboard)
Explain to him as he sounds out "bed" to look at his hands, this helped my son and after a couple of weeks he no longer struggled.
Take care,
S.