Sorry he's asleep

So our little boy has started preschool and he's also been busy with summer activites (i.e. swimming, playing with friends, etc).  On school nights he's in bed by 8:00 and other nights it's a little more laxed with a bedtime between 9:00-9:30.  Hubby has been gone on a fire for almost 2 weeks.  He's a hour ahead of us so when he calls it's 9:30 our time.  Unfortunately due to our son being in school and busy during the day, he's been falling asleep fairly early and he goes down pretty fast.  I feel so bad, that the last 3 nights when hubby has called home, he's been asleep.  I know hubby wants to talk to our son, but what can I do to make him stay awake.  A three-year-old active little boy falls asleep when his body is tired.  I feel so sad to say "sorry he's asleep" as I can sense the saddness in his voice.  I can't wait until he's home to be with us in person and then he can hear laughter and chatter from our little guy.