Hi there
You might find putting him to bed later may help. So rather than 6.45 to 7pm go for a clear 7pm, then 7.15, wait a couple of days and if WU doesn't move try 7.30, wait a couple of days again. Once wake up moves later you should then be able to shift BT a touch earlier each night, so from 7.30 go to 7.20 then 7.10 and back to 7pm.
Also make sure your first nap is not coming too early, you will need to extend the A time to where the nap 'should' be for the correct WU time.
Basically you are moving the entire day on by about 30-45 mins again (over a number of days, not in one jump) until the body clock works out to WU later in the morning.
Good luck