He sleeps until 11-12 after going to bed at 7? I would worry and I wouldn't let him sleep that long! I've never heard of a baby sleeping that long!! Both mine sleep through and occasionally ds1 whose 2 will sleep till 9
I would make sure the room is not completely dark in the mornings, I'd let some daylight in to wake up him up gradually. I find waking up in a blacked-out room very difficult and sleep twelve hours straight, which makes me feel absolutely exhausted during the day. Otherwise, if he's developing fine and has no health problems, I don't see why this should be a problem. Some adults only need four hours sleep a night and others twelve, it's no different for babies, some need more and some less.
Cabbageleaf he doesnt use blackouts just a normal cutain, ive also spoken to HV and she says he fine at the moment but hes now got an appointment to see if its a medical issue that makes him so tired. So we will see