Sudden early morning waking

My lovely mummies I'm after some advice. Roughly 4 weeks ago we had put my 30month old son to a bed, due to him climbing out of his cot. He is not bad settling down in the evening however started to get up at around 5am. This is a shock to our system as he used to get up at 7. As soon as I could hear him getting out of bed putting the light on i would put him back saying it's not time to get up yet. He would then be quiet to 30-45 minutes. To be honest I'm not sure if he is just laying in his bed or actually sleeping. We also have a gro clock to help him understand night and day. I do think he understands the clock but chooses to ignore it. We also have a 5 year old so don't want him to wake her up. He starts calling for me at around 6 at which point I go to his bedroom and stay there laying in his bed. We only leave his bedroom when the sun is out at 7am. Had anyone got any advice? Both me and my husband work shifts and we are finding it difficult to function on 4hr sleep at times