Early morning is when the pressure to sleep is lowest, so if you get a wake up after about 4am it's always harder to resettle for this reason. Remember earlier I talked about the third nap of the day always being hard? That's the same reason, baby's natural body clock is giving the least amount of pressure to go to sleep at this time.
So for the third nap, I advised a nap on the move (pushchair/car), you need to think in a similar way for early morning wakes. Accept that it will be harder to get baby back to sleep and so offer extra help. No point just doing your normal (say, dummy reinsert and hand on chest, or whatever you do), it will need extra.
Lots of families get baby back to sleep in early mornings by bringing baby into their bed and cuddling/cosleeping back to sleep. Or just accept that significantly more work/help needed to get back to sleep.
Obviously the ideal is no wake up at all in the early morning, so these suggestions are ways to deal with a wake when it happens. Simultaneously work on ways to reduce the wake up at all, which then resolves the issue.
Is it hunger/thirst? Strategically choosing to reintroduce an feed when you go to bed may help this, while daytime calories catch up.
A Wake-Reset at 11pm ish might help, even without a feed. The idea is to wake baby when you go to bed. Do a clean nappy (and feed, but fully awake feed, if you're doing one). So that it's like resetting bedtime, but a few hours later. Clean nappy means that by early morning baby is more comfortable without a very full nappy. Aim to keep baby as awake as possible during this wake up, then resettle back to sleep to reset the night.
Daytime sleep routine often affects nights. If baby is overtired (or indeed undertired) then early mornings are the most likely time to wake because it's when sleep is lightest. It might be that you need to tweek your daytime sleep routine.
Method of going to sleep will also make baby more likely to wake when in a light sleep. Consistently going to sleep where he stays asleep, with minimal movement where possible. This will help baby stay in the light sleep phase of early mornings, rather than waking up.
Is it environmental? Light bleeding in from around the blind, noise from the heating/home, outdoor sounds from an open window, too hot/cold by the early morning.
Has baby had enough night sleep? If everything possible just cannot get baby back to sleep, it might be that shifting bedtime later is what is needed and accepting that, for now (it will change again), baby needs less nighttime sleep