Advice please

My son is almost 12 months. He will be one next week. The pass 2-3 weeks he been going to sleep regular time at 8pm. He will sleep for about an hour or so than wake up wide up. Like completely awake. And he won’t go to bed until 3-4am. Since he doesn’t go to sleep at night time he doesn’t wake up until 11am-12pm the next day because he’s so exhausted . his pediatrician told me to let him sleep. But a one year old waking up for the day at 12pm sometimes 12:30 pm feels wrong to me

I was wondering if anyone had any advice for me