If he is always going back to sleep after the 5am feed, you need to wake at 7am and start your day from there.
Either first feed will be a top up because he has taken a full feed at 5am or you can try and limit the feed at 5am (so it sees him through to 7am) and give a full feed at 7am.
Over the while you can try and go to a 3.75hr routine, so hopefully you will be on 4hr by the time you go back to work.
You at least have a couple of months to get him to 4hrs and the likelyhood is that he will probably want to do it himself anyway by then.
DS didn't stop his 5am feed until he was 5months.